clin chem - serum enzumes ch 19

injury to tissue releases what

releases enzymes that are used as plasma markers of tissue damage

for a substance to serve as a biochemical markers of damage to a specific organ or tissue, it must

it must arise predominantly from the organ or tissue of interest

some enzymes are found predominantly where

in specialized tissue (e.g., lipase in the pancreas) while others are more widely distributed but can have tissue-specific isoenzymes or isoforms (e.g., the pancreatic isoenzyme of alpha-amylase) that are evaluated to enhance tissue and organ specificity

isoenzymes

different forms of the same enzyme that catalyze the same reaction but have distinct properties, such as different kinetic parameters or tissue localization, arising from variations in amino acid sequence or different genes. 

isoforms

different versions of a protein or RNA molecule that arise from the same gene, often through alternative splicing or other post-transcriptional modifications, leading to variations in their structure and function

clinical laboratorians are principally concerned
with changes in activity in the serum or plasma of enzymes
that are predominantly intracellular and physiologically
present in the blood at low activity concentrations onl

changes in the serum activities of these enzymes are used
to infer the location and nature of pathological changes in
tissues of the body

Therefore, an understanding of the factors that affect the rate
of release of enzymes from their cells of origin and the rate at
which they are cleared from the circulation is necessary to
interpret correctly changes in activity that occur with disease

Knowledge of the intracellular locations of enzymes assists in
determining the nature and severity of a pathological process

– a mild, reversible viral inflammation of the liver, such as a
mild attack of viral hepatitis, is likely to increase only the
permeability of the cell membrane, thereby allowing
cytoplasmic enzymes to leak out into the blood

– a severe attack causing cell necrosis also disrupts the
mitochondrial membrane, and both cytoplasmic and
mitochondrial enzymes are detected in the blood

diagnostic window for an injury marker is defined as

defined as the interval of time after an episode of injury during which
plasma concentrations of the marker are increased, thereby
demonstrating the occurrence of injury
• Some of the main enzymes of established clinical value,
together with their tissues of origin and their major clinical
applications, are listed in Table 19-1

creatine kinase (CK or CPK)

dimeric enzyme (82 kDa) that catalyzes the reversible phosphorylation of creatine (Cr) by adenosine triphosphate (ATP)

physiologically, when muscle contracts, what happens

ATP is converted to adenosine diphosphate (ADP) and CK catalyzes the rephosphorylation of ADP to ATP using creatine phosphate (CrP) as the phosphorylation reservoir

CK activity in the serum of healthy ppl

• due almost exclusively to CK-MM activity (although small amounts of CK-MB may be present) and is the result of physiological turnover of muscle tissue

CK-MB assays, URL (upper reference limit) for males and females are

URL for males 5.0 micrograms/L, with values for females being less than male values

most clinically important liver enzymes include

1) alanine aminotransferase

2 aspartate aminotrasnferases

3) gama-glutamyltransferase

4) alkaline phosphatase

5 5’ nuucleotidase

clinically the most common alterations in liver enzyme activities are associated with

(1) hepatocellular damage (elevated aminotransaminase
activities) and (2) cholestasis (elevated alkaline phosphatase, 5′-
nucleotidase, and γ-glutamyltransferase activities)

pyridoxal-5’- phosphate (P-5’ -P) and its amino analogue, pyridoxamine-
5′-phosphate, function as coenzymes in amino-transfer reactions

The P-5′-P is bound to the apoenzyme and serves as a true prosthetic group, whereby it accepts the amino group from the first substrate—aspartate or alanine—to form enzyme-bound pyridoxamine-5′-
phosphate and the first reaction product, oxaloacetate or pyruvate,
respectively

– the coenzyme in amino form then transfers its amino group to the
second substrate, 2-oxoglutarate, to form the second product,
glutamate, thereby regenerating the P-5′-P

aminotransaminases are widely distributed throughout the body. where is it found ?

AST is found primarily in the (1) heart, (2) liver, (3) skeletal muscle, and (4)
kidney, whereas ALT is found primarily in the liver and kidney, with lesser
amounts in heart and skeletal muscle

ALT is exclusively cytoplasmic; however, genetically distinct mitochondrial
and cytoplasmic isoforms of AST exist, both with a dimeric structure
composed of two identical polypeptides of about 400 amino acids

liver disease is the most important cause of increased aminotransaminase activity in serum

in most types of liver disease, ALT activity is higher than that of AST
– Exceptions may be seen, however, in (1) alcoholic hepatitis, (2)
hepatic cirrhosis, and (3) liver neoplasia
– In viral hepatitis and other forms of liver disease associated with
acute hepatic necrosis, serum AST and ALT activities are elevated
even before the clinical signs and symptoms of disease (e.g.,
jaundice) appear, and activities for both enzymes may reach values
as high as 100 times the upper reference level (URL), although 10-
fold to 40-fold elevations are most frequently encountered

Although serum activities of both AST and ALT become elevated
whenever disease processes affect liver cell integrity, ALT is the more
liver-specific enzyme

Thus, serum elevations of ALT activity are rarely observed in conditions other than parenchymal liver disease, so the incremental benefit of determination of AST, in addition to ALT, may be limited in the assessment of liver disease

after acute myocardial infarction and hemolytic disease

Increased AST activity appears in
serum, as well as in chronic diseases like progressive muscular dystrophy
and dermatomyositis, although it is usually normal in other types of
muscle disease, especially in those of neurogenic origin
• Slight to moderate AST elevations are also noted in hemolytic disease

hemolyzed specimens should not be used

• especially when AST is measured, because of the large amount of this enzyme present in red cells

• There are no significant sex-related differences in AST activity, but
  a difference in ALT activity has been noted between adult males
  and females
  • ALT does not reveal an age dependency during childhood, whereas
  serum AST activity in neonates and in children younger than 3 years old is twice that in adults

what do peptidases do?

Peptidases catalyze the hydrolytic cleavage of peptides to form amino acids or smaller peptides and they constitute a broad group of
enzymes of varied specificity, with some individual enzymes acting as
amino acid transferases that catalyze the transfer of amino acids from
one peptide to another amino acid or peptide

gamma-glutamyltransferase (GGT)

GGT catalyzes the transfer of the γ-glutamyl
group from peptides and compounds to an acceptor. Substrates for
GGT include (1) the γ-glutamyl acceptor, (2) some amino acid or
peptide, or (3) even water, in which case simple hydrolysis takes place

The enzyme acts only on peptides or peptide-like compounds that
contain a terminal glutamate residue joined to the remainder of the
compound through the terminal (γ-) carboxyl