Disorders of Amino Acids Metabolism

Most of these diseases are rare and unlikely to be encountered by most practicing physicians.

If left untreated, many of these genetic disorders result in irreversible brain damage and early mortality.

Prenatal or early postnatal detection and rapid initiation of appropriate treatment, if available therefore are essential.

Since several of the enzymes concerned are detectable in cultures of amniotic fluid cells, prenatal diagnosis by amniocentesis (the surgical insertion of a hollow needle through the abdominal wall and into the uterus of a pregnant female to obtain amniotic fluid esp. to examine the fetal

chromosomes for an abnormality) is possible.

Mutations in the exons or in the regulatory regions of a gene that encodes an enzyme of amino acid catabolism can result in a nonfunctional enzyme or in complete failure to synthesize that enzyme.

The modified or mutant enzyme may possess altered catalytic efficiency (low Vmax or high Km) or altered ability to bind an allosteric regulator of its catalytic activity·

Treatment consists primarily of feeding diets low in the amino acids whose catabolism is impaired.

Aminoaciduria:

Amino acids in blood are filtered through the glomerular membrane but are normally reabsorbed in the renal tubules by saturable active transport mechanisms.

Three types of aminoaciduria may be distinguished:

(I) Overflow aminoaciduria, in which the plasma level of one or more a.a exceeds the renal threshold.

(2) Renal aminoaciduria, in which plasma levels are normal but there is a congenital or acquired defect of the renal transport system.

(3) No-threshold aminoaciduria, which is similar to the overflow type except that plasma levels are essentially normal. Example is homocystinuria that are not due to congenital or acquired kidney defects but are solely due to inefficient reabsorption by normal renal tubules

Aminacidurias may be primary or secondary:

a) Primary aminoaciduria: due to an inherited enzyme defect, also called inborn error of metabolism. The defect is located either in the pathway by which a specific a.a is metabolized or in the specific renal tubular transport system. It should be possible to diagnose such an inherited disease at 3 levels: (I) The DNA abnormality. (2) The enzyme defect. (3) The metabolic abnormalities due to the defect. b) Secondary aminoaciduria: due to a disease of an organ such as the liver, which is an active site of a.a metabolism, or to generalized renal tubular dysfunction, or to protei~-energy malnutrition.

Some metabolic disorders of amino acids

1- Glycine metabolic disorders:

a) Glycinuria: Cháracterized by uòinary excretion of 0.6-1 g of glycine

per day and a tendency to form oxalate renal stones. Glycinurie probably results from a defect in renal

tubular reabsorption since plasma glycine levels are normal.

b) Primary Hypezoxaluria: In primary hyperoxaluria, urinar{ excretion of oxalate

is unrelated to dietary intake of oxalate. The oxalate apparently arises from deamination of

glycine, forming glyoxylate (oxalate semialdehyde). The metabolic defect involves a failure to catabolize

glyoxylate, which therefore is oxidized to oxalate:

Glycine Glyoxylate CO2 + H2O

Oxalate Ca oxalate crystals

Clinical features include progressive bilateral calcium oxalate urolithiasis, nephrocalcinosis, and recurrent infection of the urinary tract that are followed by early mortality from renal failure or hypertension.

Treatment involves administration of pyridoxal phosphate (vit B6) which is a co-transaminase that stimulates the transamination of glyoxylate again to glycine.

2- Sulphur-containing amino acids metabolic disorders:

a) Cystinuria (Cystine-Lysinuria): This inherited metabolic disease. It is characterized by urinary excretion of cystine up

to 30 times normal. Excretion of lysine, arginine, and ornithine also rises. The defect is in the renal reabsorptive mechanisms

for these 4 amino acids. Since cystine is relatively insoluble, cystine calculi

form in the renal tubules of cystinuric patients. Treatment involves reducing the concentration of

cysteine in urine by drinking large amounts of water, increasing the solubility by alkalinization of urine.

b)Cystinosis (Cystine storage disease) : Cystinosis is a rare lysosomal disorder. The defect is in the carrier-mediated transporter of

cystine. Cystine crystals are deposited in tissues and organs,

particularly the reticuloendothelial system. Cystinosis usually is accompanied by a generalized

aminoaciduria. Other renal functions are also seriously impaired,

and patients usually die young from acute renal failure.

c) Homocystinurias:

NADPH + O2

Phenylalanine hydroxylase

Dihydrobiopetrin

It is a heritable defects of methionine catabolism. The defect is due to deficiency of cystathionine

synthase (CS):

Transmethylation CSMethionine Homocysteine Cystathionine (SAM) + Serine

Incidence is 1:160.000 births. Up to 300 mg of homocysteine, some together with

S-adenosylmethionune (SAM), is excreted daily in urine. Plasma methionine levels are also increased.

Clinical features include mental retardation, osteoporosis, thrombosis and dislocated lenses.

Treatment is by reducing methionine in diet and administration of pyridoxine.

3- Phenylalanine and tyrosine metabolic disorders:

a) Phenylketonuria (PKU or phenylalaninemia):

The major metabolic disorders associated with the impaired ability to convert phenylalanine to tyrosine:

Phenylalanine Tyrosine

The defects in the conversion pathway may be due to:i- Absence or decreased phenylalanine hydroxylase

(classical PKU or type I).ii- Defects in dihydrobiopetrin reductase (Types II,

III).iii- Defects in dihydrobiopetrin biosynthesis (Types

IV,V).

HomogentsateOxidase

The major consequence of untreated Type I PKU is mental retardation, seizures, eczema a mousy odour.

Laboratory findings include increased phenylalanine metabolites (phenylpyruvic, phenylacetic and phenyllactic acids) in blood and urine. Blood tyrosine does not rise after phenylalanine load.

Treatment is by restriction of phenylalanine in die is by restriction of phenylalanine in diet.

Tyrosinemia (Tyrosinosis):

Tyrosine p-hydroxyphenylpuruvate Homogentsic acid

Maleylacetoacetate Fumarylacetoacetate

Fumarylacetoacetate hydrolase

Fumarate + Acetoacetate

Defect : Type I: deficiency of fumarylacetoacetate hydrolase.. Type II: deficiency of tyrosine transaminase

Incidence is 1:100 000. Laboratory findings include excess tyrosine and its metabolites p-hydroxyphenylpyruvic, acetic & lactic acids.

The clinical features are hepatic cirrhosis, renal damage that may cause ammonia intoxication & renal glucosuria.

TreatmentTreatment: dietary restriction of phenylalanine, tyrosine & methionine

Alkaptonuria:

This is an inherited metabolic disorder.

Tyrosine Transaminase

The defect is due to the absence of liver homogentisic acid oxidase.Incidence is 1:250 000.There is an increased blood homogentisic acid that passes in urine. Urine becomes brown-black on standing.Clinical features: degenerative arthritis, cartilage pigmentation Treatment: non available.

d) Hartnup disease: It is an autosomal recessive trait.The defects are in the intestinal and renal transport of

neutral amino acids, including tryptophan. Signs include

i. A general neutral aminoaciduria.ii. Increased excretion of indole derivatives

that arise from intestinal bacteria degradation of unabsorbed tryptophan.

iii. The impaired intestinal absorption and renal reabsorption of tryptophan limits the tryptophan available for niacin biosynthesis and accounts for the accompanying pellagra-like signs and symptoms.

4- Metabolic disorders of branched-chain amino acids metabolism: (Valine, Leucine, and Isoleucine):

- Maple syrup urine disease (Branched chain ketonuria):

It is a heredity autosomal recessive disorder. Incidence is 1:185.000 world wide). Biochemical defect is the absence or greatly reduced

activity of the –ketoacid decarboxylase

Valine, Leucine, Isoleucine

Transamination

Corresponding –keto acids

–ketoacid decarboxylase

CO2 + Corresponding Acyl CoA

Valine Isoleucine Succinyl Leucine Propionyl CoA CoA + Acetyl CoA

HMG CoA

Plasma and urinary levels of valine, leucine, isoleucine and their –keto acids are elevated.

The odour of the urine is that of maple syrup or burnt sugar.

The disease is evident by the end of first week of extra uterine life.

The infant is difficult to feed and may vomit. Extensive brain damage occurs in the surviving

children. Without treatment, deadh usually occurc by the end of 1 year.

Diagnosis prior to one week of age is possible only by enzymatic or genetic analysis.

Treatment initiated in the first week of life largely averts alleviates consequences.

Therapy involves replacing dietary protein by a mixture of amino acids that exclude valine, leucine, and isoleucife. Whej plasma lavals of these aminn a#ids fall po normal, thay ara restored in the form ob milk and other food in !mounts that never exceed metabolic demand&

Plasma Proteins Abnormalities

Functions of plasm` propeinc:

Dhe plasma proteijs are m/stly synthesized an the hepatmcytes, `xcept for the immunnglobulins shhbh ard sylt`esized il t`e lxep`opeticelaR systel.

The prilciple functiofs of the plasma propehns `be:

(I) Maintelance of colloidal oncotic pressure, a functiol mainly of albuein.

(2) Tbansporter or binding proteinc e.g.:

a) Essential metabodites : e.g. lipids (as lipoproteins).b) Hormones : e.g* thyroxine (thyroid binding globulin)

cortisol (transcortin).c) Metals . e.g. transderrin (transport iron) and

cdruloplasmin (contains Cu2+).d) Excretory products : e.e. bilirubin (bound to albumin).e) Drugs and various toxic ingested substances.

(3) Defenre reactions, functions whhch depend ona) immunoglobulins and

b) the complement system.

(4) Coagulation and fibrinolysis, which involve some proteins circulating in plasma and others liberated, for instance, from damaged platelets.

(5) Buffering of H+.

Total Protein (DP):

Nopmal leral jf plasma total protdin is 6-8 g'dl. Total protein estim`tion is ob limited clinical value. Acupe change1 in concentration tsualdy reflect the r!

tio of proteils to fluids in the vascular compartment, rather than Change in protein sqnthesis.

Acute changeq are usually due tm:

1. Doss or gaan of prntehn-f2ee fluids, such as in dehydratioj.

2. Marked changes o& major constituents, such as albumin & immunnglobul)fs.

Plasma totad protein concentration may be misleading: For example:

Normal [TP] albumin immunoglobulin

The Concentration of Protein in Plasma Is Important In Determining the Distribution Of Fluid Between Blood And Tissues:

Methods of investigation of plasma proteins : 1.2.2. Direct chemical measurements, e.g. biuret method for , e.g. biuret method for

detecting the presence of peptide bonds. This detecting the presence of peptide bonds. This measures the total concentration of proteins, single or measures the total concentration of proteins, single or in a mixture.in a mixture.

3.3. Direct physical measurements , e.g. albumin by dye , e.g. albumin by dye binding.binding.

4.4. Measurement after separation by techniques such as by techniques such as electrophoresis or isoelectric focusing.electrophoresis or isoelectric focusing.

Measurement of biological activity e.g. enzymese.g. enzymes..Immunological methods..

Serum protein electrophoresisSerum protein electrophoresis:

This technique separates proteins mainly on the basis ofThis technique separates proteins mainly on the basis of their electric charge. It can be performed on a variety of their electric charge. It can be performed on a variety of materials e.g. paper cellulose acetatematerials e.g. paper cellulose acetate:

Polyacrylamide gel electrophoresis (PAG) or starch gel electrophoresis commonly yields 20 or more fractions whose principle components are:

1- Pre-albumin .2- Albumin .3- -Globulins contain Lipoprotein (HDL), -Fetoprotein, -

Antitrypsin, -Acidglycoprotein, Prothrombin.4- -Globulins contain Thyroxine-binding globulin, Ceruloplasmin,

Heptaglobulin, -Macroblobulin.

5- -Globulins contain Transferrin, Plasminogen, -lipoprotein (LDL)

6- -Globulins include: IgG, IgM, IgA, IgD, IgE.

Albumin:

Normal level: 3.4-4.7 g/dl Albumin is the major plasma protein of human plasma

about 60% of total plasma proteins (6-8 g/dl). The serum albumin/globulins ratio (A/G ratio) is normally

2/1. Albumin is synthesized in liver and its half-life time is

about 20 days.

Its chief biological functions are:1. Transport and store a wide variety of ligands (such as

bilirubin, long chains FA's, and many hormones e.g. thyroxine, T3, cortisol and aldosterone).

2. Maintain the plasma oncotic pressure. 3. Serve as a source of endogenous amino acids.

Serum albumin is increased in: Dehydration or i.v. albumin administration. Serum albumin is decreased in:

a- Liver disease both acute and chronic but reduction in plasma albumin may not be detected in acute liver disease because of the half-life time of albumin.

b- Malnutrition .c- Increased need e.g. in pregnancy.d- Malabsorptive disease.e- Increased catabolism as a result of injury e.g trauma,

major surgery or infection.f- Increased loss as in burns and nephrotic syndrome.

In severe hypoalbuminemia , when plasma albumin level is below 2.5 g/dl, the low plasma oncotic pressure allows water to move out of blood capillaries into tissue causing edema.

Pre-albumin

It is normally present in plasma in small amounts. It is synthesized in hepatocytes. It functions as one of the transport proteins for vitamin A

and thyroxine. The plasma pre-albumin falls rapidly in response to injury

and it is decreased in both acute and chronic liver disease because its half-life time is much shorter than albumin.

Transferrin It is a 1-globulin and it’s a glycoprotein.

It is the major iron-binding protein in plasma, transports iron from the sites of absorption and red cells breakdown to the developing red cells in the bone marrow.

Acquired deficiency occurs in protein-losing conditions, infection and neoplastic disease.

Increased plasma transferrin occurs in iron deficiency states and in women taking estrogen-containing oral contraceptives.

The Haptoglobulins This is a group of proteins, all 2-globulin that binds

hemoglobin to form haptoglobulin/hemoglobin complexes; the complexes are then rapidly broken down in the lymphoreticular system.

Whenever intravascular hemolysis is increased, plasma haptoglobulin falls and free haptoglobulin is undetectable as in:

1. Hemolytic anemia. 2. Liver disease.

Plasma haptoglobulin is increased in acute infections, in the nephrotic syndrome and following trauma (an injury as a wound to living tissue caused by an extrinsic agent ‹surgical trauma›).

Ceruloplasmin It is an 2-globulin that has a blue color because of high

copper content and carries 90% of the copper present in the plasma.

Function : Ceruloplasmin is primarily may be concerned with either:

1. Plasma copper transport, or 2. Its enzyme properties as an oxidase are of

physiological importance.

Plasma ceruloplasmin is reduced in patient with malnutrition, and in nephrotic syndrome.

Plasma ceruloplasmin is increased during pregnancy and in women taking estrogen-containing oral contraceptives, also in acute infections and some types of liver disease.

1 Fetoprotein (AFP) This protein is present in the tissues and plasma of the

fetus. Its concentration falls very rapidly after birth, but minute

amounts can still detected in plasma of adults. The function of AFP is unknown but its measurement has

important applications in investigation of disease in adults:1. Pregnant women carrying fetuses with open neural

tube defects have raised levels of plasma AFP.2. Gross increase in serum AFP occurs in about 50% of

patients with hepatocellular carcinoma, and AFP is used as a tumor marker.