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Case Studies

A 56-year-old man with long-standing poorly controlled diabetes mellitus visits his primary care physician for a follow-up after recent hospitalization. The patient experienced an episode of acute renal failure while in the hospital and his creatinine level rose to 4 mg/dl. Creatinine, a marker of renal function is produced from which of the following precursors?

a) Glutamine, Aspartic acid and alanine

b) Glutamine, Cysteine and Glycine

c) Serine, Glycine and Methionine

d) Glycine, Arginine and Methionine

e) Glutamic acid, Cysteine and Glycine

Creatinine is an anhydrous product of creatine. Creatine, methyl guanidoacetic acid, is a tripeptide produced from Glycine, Arginine and Methionine in the human body primarily in the kidney and liver. It is transported in the blood for use by muscles. Approximately 95% of the total creatine is located in skeletal muscle. It is also present in liver, testes and brain. It can occur in free form or phosphorylated form. The phosphorylated form is called ‘Creatine-Phosphate’, ‘Phosphocreatine’ or ‘phosphagen’.

Creatine is excreted in the form of Creatinine, which is formed by removal of one molecule of water from creatine.

Creatine————–> Creatinine + H20

The reaction is non enzymatic and irreversible. About 2 % of the total Creatine is converted daily to creatinine so that the amount of creatinine produced is related to the total muscle mass and remains approximately the same in plasma and urine from day to day unless the muscle mass changes. Normal excretion ranges between 1-2 g/day.

Estimation of serum and urinary creatinine is undertaken to assess the renal functions.

The normal serum creatinine ranges as follows-

1) < 12 Years-0.25-0.85 mg/dL

2) Adult male- 0.7-1.5 mg/dL

3) Adult Female- 0.4 -1.2 mg/dL

It is higher in males because of the muscle mass.

General discussion of peptides

Peptides are short polymers of amino acids. Physiologically they perform prominent roles  in the neuroendocrine system as hormones, hormone-releasing factors, neuromodulators, or neurotransmitters. Clinically some of the peptides are used as antibiotics, anticancer drugs and some in toxic dosage promote tumor formation.

Summary of biologically important peptides (Alphabetically)

S. No. Name of Peptide Number of amino acids Physiological/ Clinical Significance
1) Angiotensins

a) Angiotensin I

b) Angiotensin II

c) Angiotensin III

 

10

8

7

Angiotensins are produced from Angiotensinogen . Renin, an enzyme produced in the juxtaglomerular cells of the renal afferent arteriole acts upon the substrate angiotensinogen to produce the decapeptide angiotensin

I.Angiotensin-converting enzyme, a glycoprotein found in lung, endothelial cells, and plasma, removes two carboxyl terminal amino acids from the decapeptide angiotensin I to form angiotensin II. Angiotensin II is the most potent vasoconstrictor of the body, it  also stimulates release of Aldosterone.Angiotensin II is converted to the heptapeptide angiotensin III, an equally potent stimulator of aldosterone production. In humans, the plasma level of angiotensin II is four times greater than that of angiotensin III, so most effects are exerted by the octapeptide.

2) Aspartame 2 Used as artificial sweetener
3) Bradykinin 9 It is a natural smooth muscle relaxant
4) Bacitracin Cyclic polypeptide Bacitracin is commercially manufactured by growing the bacteria Bacillus subtilis. It is used as a topical antibiotic .
5) β- Corticotropin (ACTH) 39 ACTH stimulates secretion of Glucocorticoid hormone from adrenal cortex cells, especially in the zona fasciculata of the adrenal glands.
6) β- MSH 18 Melanocyte-stimulating hormone (MSH) belongs to a group called the melanocortins. This group includes ACTH, alpha-melanocyte-stimulating hormone (α-MSH), beta-melanocyte-stimulating hormone (β-MSH) and gamma-melanocyte-stimulating hormone (γ-MSH); these peptides are all cleavage products of a large precursor peptide called pro-opiomelanocortin (POMC).They stimulate the production and release of melanin (melanogenesis) by melanocytes in skin and hair.
7) Bleomycin Polypeptide It is produced by the bacterium Streptomyces verticillus. Acts as an anticancer drug used in the treatment of Hodgkin’s lymphoma, testicular tumors and squamous cell carcinoma.
8) Creatine/Creatinine 3 Energy bank, stored as creatine-P in skeletal muscles, a high energy compound.
9) Carnosine 2 It is highly concentrated in muscle and brain tissues. Anserine is methylated Carnosine.
10) Dynorphin 13 Dynorphin , endogenous opioid peptide has been shown to be a modulator of pain response. Also called super opiate since it is highly potent.
11) Endorphins 5-40 Endorphins (“endogenous morphine”) are endogenous opioid peptides that are not considered to be neurotransmitter molecules, but are instead classified as neuromodulatory, that is, they modify the action of neurotransmitters through a number of effects associated with pain or pleasure.The term endorphin consists of two parts: endo- and -orphin; these are short forms of the words endogenous and morphine, intended to mean “a morphine-like substance originating from within the body. They resemble the opiates in their abilities to produce analgesia and mood elevation.
12) Gastrin Gastrin is found primarily in three forms:a) gastrin-34 (“big gastrin”)b) gastrin-17 (“little gastrin”)c) gastrin-14 (“minigastrin”)d) Also, pentagastrin is an artificially synthesized, containing  five amino acids. The presence of gastrin stimulates parietal cells of the stomach to secrete hydrochloric acid (HCl)/gastric acid.
13) Glutathione 3 Glutathione is commonly abbreviated as GSH (because of the sulfhydryl group of its cysteine, which is the business part of the molecule).It participates in the decomposition of potentially toxic hydrogen peroxide in the reaction catalyzed by glutathione peroxidase.It is an important intracellular reductant, helping to maintain essential SH groups of enzymes in their reduced state. Also helps in the transport of certain amino acids across membranes in the kidney.
14) Gramicidin S 10 (Cyclodecapeptide) Gramicidin S is an antibiotic effective against some Gram positive and Gram negative bacteria as well as some fungi.
15) Glucagon 29 (Polypeptide) Regulated blood glucose level by causing glycogenolysis and gluconeogenesis. Also causes lipolysis and reduces protein synthesis.
16) Kallidin 10 Kallidin is a bioactive kinin formed in response to injury from kininogen precursors through the action of kallikreins.Kallidin is a decapeptide that can be converted to bradykinin by the aminopeptidase enzyme. Kallidin is identical to bradykinin with an additional lysine residue added at the N-terminal end.
17) Leuenkephalin 5 Produces opioid effects, such as analgesia and mood elevation.
18) Met enkephalin 5 [Met]enkephalin, sometimes referred to as opioid growth factor (OGF), is a naturally occurring, endogenous opioid peptide that has opioid effects of a relatively short duration. [Met]enkephalin produces its opioid effects, such as analgesia and mood lift through binding to specific receptors.
19) Microcystin Cyclic peptide Hepatotoxic
20) Nodularin 5 Nodularin-R is a potent hepatotoxin and may cause serious damage to the liver.
21) Oxytocin 9 It plays a role in milk ejection from the lactating mammary glands(‘Let down reflex’)  and also promotes  uterine contractions, important for cervical dilatation during the second and third stage of labor.
22) Penicillin 3 Penicillin is a group of antibiotics derived from Penicillium fungi. The term “penicillin” is often used generically to refer to benzylpenicillin (penicillin G), procaine benzylpenicillin (procaine penicillin), benzathine benzylpenicillin (benzathine penicillin), and phenoxymethylpenicillin (penicillin V).
23) Polymyxins These are cyclic, positively charged peptides Polymyxins are antibiotics; they are used in the treatment of Gram-negative bacterial infections. Polymyxin antibiotics are relatively neurotoxic and nephrotoxic and are usually used only as a last resort if modern antibiotics are ineffective or are contraindicated.
24) Secretin 27(Polypeptide) Secretin stimulates the release of watery bicarbonate rich fluid  from pancreatic and bile duct epithelium.
25) Vasopressin 9 Arginine vasopressin (AVP), also known as vasopressin, argipressin or antidiuretic hormone (ADH), is a peptide hormone that controls the reabsorption of water in the tubules of the kidney by affecting  the cellular permeability. It also increases peripheral vascular resistance, which in turn increases arterial blood pressure. It plays a key role in the maintenance of water and electrolyte balance of the body.

 

Functional classification of biologically important peptides (Easy to remember)

1) Peptide hormones

i) β- Corticotropin (ACTH)

ii) β- MSH

iii) Gastrin

iv) Glucagon

v) Secretin

vi) Oxytocin

vii) Vasopressin

2) Antibiotics

i) Bacitracins

ii) Penicillin

iii) Polymyxins

iv) Gramicidin S

v) Chloramphenicol

3) Anticancer drug

Bleomycin

4) Muscle relaxants

i) Kallidin

ii) Bradykinin

5) Vasoconstrictors

i) Angiotensins

a) Angiotensin I

b) Angiotensin II

c) Angiotensin III

ii) Vasopressin

6) Neuromodulators

i) Endorphins

ii) Enkephalin

7) Toxic peptides

i) Microcystin

ii) Nodularin

8) Biological reductant

Glutathione

9) Artificial sweetener

Aspartame

10) Muscle peptides

i) Creatine

ii) Carnosine

iii) Anserine

 

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Case details

A severe form of obstructive lung disease starting with dyspnea and leading progressively towards emphysema was found in several members of the family. One brother had died earlier of lung disease.

Blood plasma from his surviving brother and sister showed abnormally low concentration of α1-Antitrypsin (5.5 μm, normal is 20–48 μm), the α1-plasma fraction also moved abnormally on isoelectric focusing gel electrophoresis.

What is the relationship of emphysema with α1-Antitrypsin?

What is the cause of abnormal movement during electrophoresis?

What are the prospects for treatment of this emphysema?

Case discussion

Emphysema is a lung disease characterized by destruction of the alveolar walls. Its causes are complex but airway infection, cigarette smoking, air pollution and familial factors are involved. A deficiency in plasma α1- Antitrypsin leads to the development of emphysema and less frequently hepatic disease.

α1-Antitrypsin

It was so named because it is a good inhibitor in vitro of pancreatic protease trypsin. It is synthesized primarily in the liver and is secreted in to the blood stream. The inhibitor makes-up most of the proteins in the α1-globulin band following the electrophoresis of plasma.

Pathogenesis of α1-Antitrypsin Deficiency

In healthy individuals, the lungs contain a natural enzyme called neutrophil elastase that, under normal circumstances, helps the lungs by digesting damaged, aging cells and bacteria. This process promotes healing of the lung tissue. Unfortunately this enzyme is nonspecific in nature and eventually attacks the lung tissue instead of helping it heal. That is where alpha antitrypsin (AAT) comes into action, by destroying the enzyme before it can cause actual damage to healthy lung tissue. It promotes the lungs to function normally. When there is not enough AAT, however, the lung tissue will continue to be destroyed, which may lead to the chronic lung disease, emphysema.

Inheritance

The two patients described in this case are male and female, suggesting that the deficiency is inherited as an autosomal trait. The genetics has been studied in large number of patients and family members. Analysis suggests that two independent alleles exist on chromosome 14. The deficiency is inherited as a codominant trait; that is, both alleles contribute to the disease and neither is dominant over the other. The two genes that each person carries for α1-Antitrypsin may be diverse, more than 100 variants are known.

The two common deficiency variants of a1-antitrypsin, S and Z, result from point mutations in the α1- antitrypsin gene and are named on the basis of their slower electrophoretic mobility on isoelectric focusing analysis compared with the normal M allele. S α1-antitrypsin (264Glu→Val) is found in up to 28 percent of Southern Europeans and, although it results in plasma α1-antitrypsin levels that are 60 percent of the M allele, it is not associated with any pulmonary sequelae. This is usually sufficient to protect the lungs from the effects of elastase in people who do not smoke.

The Z variant (342Glu→Lys) results in a more severe deficiency that is characterized, in the homozygote, by plasma α1-antitrypsin less than 15 percent of normal, and patients are likely to develop panacinar emphysema at a young age; 50 percent of these patients will develop liver cirrhosis, because the A1AT is not secreted properly and instead accumulates in the liver.

Clinical Manifestations

The first signs and symptoms of lung disease caused by alpha-1 antitrypsin deficiency usually appear between ages 20 and 50. The earliest symptoms are:

• Shortness of breath following mild activity,

• Reduced ability to exercise, and

• Wheezing.

Other signs and symptoms can include:

• Unintentional weight loss,

• Recurring respiratory infections,

• Fatigue,

• Rapid heartbeat upon standing.

Advanced lung disease leads to emphysema, in which small air sacs in the lungs (alveoli) are damaged.

 

Figure 1: Hyperinflation due to panacinar emphysema. Posteroanterior chest radiograph shows that the dome of the right hemidiaphragm is below the level of the anterior right 7th rib consistent with increased lung volumes. Also noted is decrease in the peripheral vascular markings. The patient had severe panacinar emphysema due to alpha-1 antitrypsin deficiency.

Characteristic features of emphysema include:

• Difficulty breathing,

• A hacking cough, and

• A barrel-shaped chest.

Smoking or exposure to tobacco smoke accelerates the appearance of symptoms and damage to the lungs.

About 10 percent of infants and 15 percent of adults with alpha-1 antitrypsin deficiency have liver damage.

Signs of liver disease can include:

• Ascites,

• Edema feet or legs, and

• Jaundice.

Diagnosis

The following tests may also help with diagnosis:

• Alpha-1 antitrypsin estimation

• Arterial blood gases

• Chest X-ray (Figure 1)

• CT scan of the chest

• Genetic testing

• Pulmonary function tests.

Measurement of alpha-1 antitrypsin: Isoelectric focusing on polyacrylamide gel is useful in distinguishing specific variants to establish the specific phenotype.

Effect of smoking on antitrypsin levels: Smoking greatly exacerbates antitrypsin deficiency. A methionyl-seryl group   (position 358 and 359) is present at the point where the inhibitor binds to the active site of elastase. The smoke promotes the oxidation of Methionine residues to the corresponding sulfoxide. This results in inactivation of the protein which fails to bind elastase . The active elastase then destroys the lung tissue.

Treatment

Treatment involves replacing the missing AAT protein. The protein derived from donated human plasma is given intravenously each week. This augmentation therapy is thought to arrest the course of the disease and halt any further damage to the lungs. Long-term studies of the effectiveness of A1AT replacement therapy are not available. It is currently recommended that patients begin augmentation therapy only after the onset of emphysema symptoms.

Quitting smoking is crucial.

Other treatments specifically for emphysema and cirrhosis are also used.

Treatments currently being studied include recombinant and inhaled forms of A1AT. Antioxidants are also supplemented.

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Case Details

An 80 –year-old man presented with impairment of brain functions, alterations of mood and behavior.His family reported that he was having progressive disorientation and memory loss over the past 6 months.  He had trouble handling money and paying bills. He repeated questions, took longer to complete normal daily tasks, had poor judgment, and had developed mood and personality changes.

There was no family history of dementia.The routine blood, urine and C.S.F analysis did not reveal much.After a computerized tomography (CT) scan and the histopathological examination of the brain tissue, the patient was diagnosed to be having Alzheimer disease.

What is the defect in this disease?

How is the diagnosis done and what is its prognosis?

Case discussion

Alzheimer’s disease

Basic Concept

Alzheimer’s disease is defined as premature aging of the brain, usually beginning in mid-adult life and progressing rapidly to extreme loss of mental powers—similar to those, seen in very, very old age.

Clinical features

Alzheimer’s disease is a progressive and fatal neurodegenerative disorder that results in impairment of a person” ability to perform routine activities and the patient finally enters in to a vegetative state with no comprehension to the outside world. The disease runs in four stages, pre dementia. Mild dementia, moderate dementia and advanced dementia.

Although the course of Alzheimer’s disease is unique for every individual, there are many common symptoms.

The earliest observable symptoms are often mistakenly thought to be ‘age-related’ concerns, or manifestations of stress. In the early stages, the most commonly recognized symptom is memory loss, such as difficulty in remembering recently learned facts.

As the disease advances, symptoms include confusion, irritability and aggression, mood swings, language breakdown, long-term memory loss, and the general withdrawal of the sufferer as their senses decline.

Gradually, bodily functions are lost, ultimately leading to death.

Incidence

 Alzheimer’s disease is the most common form of dementia in the elderly and about 5 million people in the United States are estimated to be afflicted by this disorder. The percentage of persons with Alzheimer’s disease approximately doubles with every five years of age, with about 1 percent of 60-year-old and about 30 percent of 85-year-olds having the disease.

Biochemical defect

Alzheimer’s disease has been identified as a protein misfolding disease, caused by accumulation of abnormally folded A-beta and tau proteins in the brain. Plaques are made up of small peptides, 39–43 amino acids in length, called beta-amyloid (also written as A-beta or Aβ). Beta-amyloid is a fragment from a larger protein called amyloid precursor protein (APP), a transmembrane protein that penetrates through the neuron’s membrane.

APP is critical to neuron growth, survival and post-injury repair.

 In Alzheimer’s disease, an unknown process causes APP to be divided into smaller fragments by enzymes through proteolysis. One of these fragments gives rise to fibrils of beta-amyloid, which form clumps that deposit outside neurons in dense formations known as senile plaques.(Figure-1)

AD is also considered a tauopathy due to abnormal aggregation of the tau protein. Every neuron has a cytoskeleton, an internal support structure partly made up of structures called microtubules. These microtubules act like tracks, guiding nutrients and molecules from the body of the cell to the ends of the axon and back. A protein called tau stabilizes the microtubules when phosphorylated, and is therefore called a microtubule-associated protein. In AD, tau undergoes chemical changes, becoming hyperphosphorylated; it then begins to pair with other threads, creating neurofibrillary tangles and disintegrating the neuron’s transport system.

Amyloid plaques and Neuro fibrillary tangles

Figure-1 -showing amyloid plaques and Neuro fibrillary tangles in Alzheimer disease

AD, is also associated with a decrease in the cerebral cortical levels of several proteins and neurotransmitters, especially acetyl choline; there is reduction in nor epinephrine levels also in brain stem nuclei.

ApoE 4 allele has a strong association with AD in the general population, ApoE is present in the neuritic amyloid plaques of AD, and it may also be involved in neurofibrillary tangle formation, because it binds to tau protein.

Accumulation of aggregated amyloid fibrils, which are believed to be the toxic form of the protein responsible for disrupting the cell’s calcium ion homeostasis, induces programmed cell death (apoptosis). Aβ selectively builds up in the mitochondria in the cells of Alzheimer’s-affected brains, and it also inhibits certain enzyme functions and the utilization of glucose by neurons.

Brain atrophy in Alzheimer's disease

Figure-2 showing brain atrophy

Inheritance

The vast majority of cases of Alzheimer’s disease are sporadic, meaning that they are not genetically inherited although some genes may act as risk factors. On the other hand around 0.1% of the cases are familial forms of Autosomal-dominant inheritance, which usually have an onset before age 65.

The best known genetic risk factor is the inheritance of the ε4 allele of the Apo lipoprotein E (APOE). Between 40 and 80% of patients with AD possess at least one apoE4 allele. The APOE4 allele increases the risk of the disease by three times in heterozygotes and by 15 times in homozygotes

Diagnosis

Alzheimer’s disease is usually diagnosed clinically from the patient’s history, collateral history from relatives, and clinical observations, based on the presence of characteristic neurological and neuropsychological features and the absence of alternative conditions.

Assessment of intellectual functioning including memory testing can further characterize the state of the disease.

Advanced medical imaging with computed tomography (CT) or magnetic resonance imaging (MRI), and with single photon emission computed tomography (SPECT) or positron emission tomography (PET) can be used to help exclude other cerebral pathology or subtypes of dementia.

Alzheimer’s disease is characterized by loss of neurons and synapses in the cerebral cortex and certain sub cortical regions. This loss results in gross atrophy of the affected regions, including degeneration in the temporal lobe and parietal lobe, and parts of the frontal cortex and cingulate gyrus.(Figure-2) Studies using MRI and positron emission tomography have documented reductions in the size of specific brain regions in patients as they progressed from mild cognitive impairment to Alzheimer’s disease, and in comparison with similar images from healthy older adults.

A histopathological confirmation including a microscopic examination of brain tissue is required for a definitive diagnosis. Both amyloid plaques and neurofibrillary tangles are clearly visible by microscopy in brains of those afflicted by AD.(Figure-1)

Treatment

There is no cure for Alzheimer’s disease; available treatments offer relatively small symptomatic benefit but remain palliative in nature. Current treatments can be pharmaceutical, psychosocial and care giving.

Four medications are approved by the U.S. Food and Drug Administration to treat AD. Donepezil, rivastigmine, and galantamine) are used to treat mild to moderate AD (Donepezil can be used for severe AD as well). Memantine is used to treat moderate to severe AD. These drugs work by regulating neurotransmitters. They may help maintain thinking, memory, and speaking skills, and help with certain behavioral problems. However, these drugs don’t change the underlying disease process and may help only for a few months to a few years.

Prognosis

Individual prognosis is difficult to assess, as the duration of the disease varies. AD develops for an indeterminate period of time before becoming fully apparent, and it can progress undiagnosed for years. The mean life expectancy following diagnosis is approximately seven years. The earlier the age at onset the higher the total survival years, life expectancy is part Men have a less favorable survival prognosis than women.

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