and patients are characterized by photosensitivity. Urine contains increased quantities of uroporphyrins and coproporphyrins of both types and also elevated urinary excretion of d-ALA and PBG occurs and is associated with use in serum iron.
3. Varicyate porphyria or mixed (combined) porphyria: In this neurological as well as cutaneous symptoms are seen. This is autosomal dominant. There is deficiency of portopor-phyrinogen oxidase and ferrochelatase. Clinically there is vomiting, acute attacks of abdominal pain and neuropsychiatric signs and cutaneous photosensitivity.
Bilirubin is of two types:
1. Direct bilirubin: Direct bilirubin is bilirubin diglucuronide.
It is water soluble. It is expressed as conjugated bilirubin because it can be coupled readily with Diazo Reagent (diazotized sulphanilic acid). This is the direct van den Bergh reaction.
2. Indirect bilirubin: Albumin bound bilirubin is called indirect bilirubin. It is water insoluble. It is expressed as unconju-gated bilirubin as it will not react until it is released by the addition of alcohol. The reaction with Diazo reagent after the addition of alcohol is called the indirect van den Bergh reaction.
Normal serum bilirubin level is 0.2-0.6 mg %.
Jaundice
Jaundice is due to increase in the concentration of bilirubin in the blood which imparts yellow color to the skin and conjunctive. Jaundice may be either due to over production of bilirubin than what the liver can normally excrete or a damage in liver, fails to excrete bilirubin in normal amounts.
Jaundice is of three types:
Hemolytic or Pre-hepatic Jaundice
In hemolytic jaundice, there is an increased breakdown of hemoglobin, the liver cells are unable to conjugate all the increased bilirubin formed. Increased production of bilirubin leads to increased production of urobilinogen which appears in urine in large amounts. Bilirubin will be absent in urine.
Hepatocellular or Hepatic Jaundice
This type of jaundice results from liver damage which cannot conjugate bilirubin. The indirect serum bilirubin level will be high. Urine will show the presence of bilirubin and increased amount of urobilinogen. Stool is light in color.
Biochemical changes in jaundice BiochemicalHemolyticHepatocellularObstructive investigation(pre-hepatic)(hepatic)(post-hepatic) 1.CausesDue to excessivDisease ofDue to obstruction hemolysisparenchymal cellsof biliary tract of liver 2.Serum bilirubinIncreasedIncreasedIncreased Conjugated fractionIncreasedIncreasedIncreased
(Direct) Unconjugated fraction
More than directIncreasedIncreased (Indirect)fraction(Direct form is more) van den Bergh ReactionIndirect positiveDelayed direct positiveDirect positive Urine bilirubinUsually absentPresentIncreased (but in low amount) 3.Urine urobilinogenMuch more thanNormalAbsent the normal Fecal stercobilinogenIncreasedUsually diminishedUsually diminished Serum cholesterolNormalDecreasedIncreased –Free formNormalDecreasedNormal –Esterrified formNormalMore than normalNormal Contd.
Contd. BiochemicalHemolyticHepatocellularObstructive investigation(prehepatic)(hepatic)(posthepatic) 4.Serum proteinsNormalDecreasedNormal A:G ratioNormal Decreased (Reversal of A:G ratio)
5.Prothrombin timeNormalIncreasedNormal after the
parenteral. Injection of vitamin K
6.Serum alkalineNormalNormal or moderatelyIncreased (value Phosphataseincreased (value belowabove 5 KA* units 35 KA units) 7.Serum transaminasesNormalVery high in first weekModerately raised 8.Protein floculation testNormal or weaklyusually positiveNegative except in (Thymol turbidity test)severe obstruction 9.Color of stoolDark coloredPale coloredClay colored *King Armstrong units.
Obstructive or Post-hepatic Jaundice
This type of jaundice results from the obstruction of common bile duct. As a result of obstruction, bilirubin does not pass into the intestine, so no urobilinogen is found in the urine.
Direct serum bilirubin level will be high, urine will show the presence of bilirubin. Stool is clay colored.
Physiological Jaundice or Neonatal Jaundice
Usually mild form of jaundice appears in some newborn children on the 2nd and 3rd day of life called neonatal jaundice.
Causes
1. Excessive destruction of RBCs after birth causing increased in serum bilirubin.
2. Due to hepatic immaturity
During IU life, bilirubin formed is mainly eliminiated by placenta immediately after birth where has to eliminate all the bilirubin but it is unable to deal adquately during first 10 days.
Note:
1. In infants, when serum bilirubin rises beyond 5% clinical jaundice appears.
2. Jaundice is more common and more severe is premature babies.
Phototherapy
Exposure of skin to white light converts bilirubin to a com-pound which has shorter life than bilirubin called lumirubin.
Phototherapy is used to treat infants with hemolysis.
ENZYMES
Enzymes are biological catalysts which bring about chemical reaction in living cells. They are produced by the living organism and are usually present in only very small amounts in various cells. They can also exhibit their activity when they have been extracted from the source. Enzymes are all organic compounds and a number of them have been obtained in crystalline form.
General properties of enzymes are:
1. All enzymes are proteins with exception of ribosomes.
2. Enzymes accelerate the rate of reaction by:
a. Not altering the reaction equilibrium b. Being required in a very small amount
c. By being not consumed in the overall reaction.
3. They have the enormous power for catalysis.
4. Enzymes are highly specific for their substrate.
5. Enzymes possess active sites at which interaction with substrate takes place.
6. Enzymes catalysis involves the transformation of enzyme-substrate complex as an important intermediate in their action.
7. Enzymes lower the activation energy.
8. Some enzymes are regulatory in function.
Some enzymes are purely protein in nature and depend for activity only on their structure while certain enzymes require for their function one or more nonprotein component.
They are termed as coenzymes, cofactors or prosthetic groups.
If such a compound is firmly attached to enzyme proteins then
Enzymes
C H A P T E R
6
it is called a prosthetic group. If its attachment to protein is not very firm then it is called coenzyme. Certain coenzymes exist in free state in solution and contact enzyme protein only at the times of reaction.
The term apoenzyme refers to the protein part of the enzyme.
The apoenzyme in combination with its prosthetic group (or coenzyme) constitute a complete enzyme or holoenzyme system.
Holoenzyme = Apoenzyme + Coenzyme
= Protein part + Nonprotein part Coenzymes
Many enzymes in order to perform their catalytic activity require the presence of small nonprotein molecules. Coen-zymes are low molecular weight, organic compounds, non-protein, thermostable and can be separated by dialysis.
Characteristics of Coenzymes 1. They are stable towards heat.
2. Generally derived from vitamins.
3. Function as cosubstrates.
4. They participates in:
a. Hydride (H¯) and electron transfer reactions, e.g.
NAD+, NADH, FMN, FAD, etc.
b. Group transfer reactions, e.g. CoA, TPP, pyridoxal phos-phate, tetrahydrofolic acid, etc.
Coenzymes Functions performed
NAD+, NADP+ Hydrogen transfer
FAD, FMN Hydrogen transfer
Thiamine pyrophosphate Acetyl group transfer Pyridoxal phosphate Amino group transfer
Biotin Carboxyl group transfer
Coenzyme A Acyl group transfer
Most of the coenzymes are the members of water soluble B-complex group of vitamins. Coenzymes function as the
intermediate carrier of functional groups of specific atoms or of electrons that are transferred in the overall enzymatic reactions.
Classification of Enzymes
According to the International Union of Biochemist, the enzymes are classified into six major classes.
1. Oxidoreductases: They catalyze oxidation and reduction reactions. These enzymes are divided into three groups.
a. Oxidases: Those which use oxygen as hydrogen acceptor, e.g. tyrosinase, uricase.
b. Anaerobic dehydrogenases: Those which use some other substances as hydrogen acceptor, e.g. lactic dehydro-genase, malic dehydrogenase.
c. Hydroperoxidases: Those which use hydrogen peroxide as substrate, e.g. catalase, peroxidase.
2. Transferases: They catalyze the transfer of some group from one molecule to another molecule. These enzymes are important in biological synthesis, e.g. transaminases, hexo-kinases, transacylase, transaldolase, ketolase, phosphomu-tases.
3. Hydrolases: They catalyze the hydrolysis of substrate by addition of water molecule across the bond which is split, e.g. esterases, peptidases, phosphatases, deamidases.
4. Lyases: They catalyze the addition or removal of groups from the substrate without hydrolysis, oxidation or reduc-tion, e.g. decarboxylases, carboxylase, carbonic anhydrase, aldolase, enolase, etc.
5. Isomerases: They catalyze the conversion of a compound into an isomer, e.g. racemases, epimerases, isomerases, mutases.
6. Ligases: They catalyze the linking together of molecules coupled with the breaking of pyrophosphate bound in ATP, e.g. glutamine synthetase, succinic thiokinases.
Enzyme Specificity
Enzyme specificity is determined by how well the reactant fit into the enzyme surface. Some enzymes are very specific and show activity with only one substrate. However, some other enzymes are much less particular and will catalyze reaction with similar compounds.
Generally two types of enzymatic specificities are observed in different reactions.
Zymogens: Several proteins are synthesized in inactive forms.
These are called zymogens eq. proteins digesting enzymes and blood clotting proteins. To activate zymogens, a small amount of protein is cleared from one end. This causes the protein to change shape and activate it. These changes are not reversible.
Stereospecificity
Some enzymes show specificities only with a specific group of a substrate, e.g. Urease catalyzes the hydrolysis of urea.
Alteration in the structure of urea results in the loss of activity. For example, N-methyl urea and thiourea are not the substrate for enzyme urease.
Also some enzymes show specificity towards D- and L-form of the same substrate, e.g. D-amino acid oxidase acts only on the D-form of amino acid and not on L-form.
Substrate Specificity
Some enzymes catalyzes similar type of reactions but differ in their action due to absolute substrate specificity, e.g. Pepsin hydrolyzes peptide bond involving amino group of aromatic amino acids as phenylalanine or tyrosine.
Similarly trypsin hydrolyzes peptide bond involving the carboxyl group of basic amino acids such as lysine or arginine.
FACTORS INFLUENCING THE RATE OF