Lactate isozymes

Isozymes of Lactate Dehydrogenase Are Used to Detect Myocardial Infarctions... 

Figure 7-11. Normal and pathologic patterns of lactate dehydrogenase (LDH) isozymes in human serum. LDH isozymes of serum were separated by electrophoresis and visualized using the coupled reaction scheme shown on the left. (NBT, nitroblue tetrazolium PMS, phenazine methylsulfate). At right is shown the stained electropherogram. Pattern A is serum from a patient with a myocardial infarct B is normal serum and C is serum from a patient with liver disease. Arabic numerals denote specific LDH isozymes.

Assay of plasma enzymes aids diagnosis and prognosis. For example, a myocardial infarction elevates serum levels of lactate dehydrogenase isozyme I,. 

Derm = dermal Endocr = endocrine F = female (G) = gavage (GO) = gavage in oil Gastro = gastrointestinal Hemato = hematological hr = hour(s) LD50 = lethal dose, 50% kill LDH-C4 = lactate dehydrogenase isozyme C4 LOAEL = lowest-observed-adverse-effect level M = male ... 

Plasma hormones and enzymes of testicular origin were used as markers for evaluation of acute testicular toxicity in rats treated with 1,3-DNB. Lactate dehydrogenase isozyme C4 (LDH-C4) and ABP were both elevated after treatment with doses between 10 and 25 mg/kg of 1,3-DNB (Reader et al. 1991). Testosterone levels were reduced after treatment with 10 and 32 mg/kg of 1,3-DNB (Reader etal. 1991 Rehnberg et al. 1988). 

One of the first enzymes found to have isozymes was lactate dehydrogenase (LDH) (p. 538), which, in vertebrate tissues, exists as at least five different isozymes separable by electrophoresis. All LDH isozymes contain four polypeptide chains (each of Mt 33,500), each type containing a different ratio of two lands of polypeptides. The M (for muscle) chain and the H (for heart) chain are encoded by two different genes. 

The different LDH isozymes have significantly different values of Vmax and KM, particularly for pyruvate. The properties of LDH4 favor rapid reduction of very low concentrations of pyruvate to lactate in skeletal muscle, whereas those of isozyme LDHx favor rapid oxidation of lactate to pyruvate in the heart. 

Nucleotidase present in 48,000 X Q supernatant fractions of rat and guinea pig skeletal muscle extracts has been examined briefly (7-4). 5 -UMP seems to be the preferred substrate. The enzyme from fish skeletal muscle has also been studied (75). This enzyme hydrolyzes all ribo-and deoxyribonucleoside 5 -phosphates (except dCMP and dTMP) with preference for 5 -IMP and 5 -UMP. The enzyme is strongly activated by Mn2+ Mg2+ is a less powerful activator, and Zn2+ and EDTA are inhibitors. This enzyme thus appears similar to the soluble activity from mammalian liver (88, 86). 5 -Nucleotidase in mammary gland hydrolyzes all 5 -ribonucleotides and shows a decrease from pregnancy to early lactation (76). Rats injected with glucagon show increased 5 -nucleotidase in pancreatic islet tissue (77). The enzyme in mouse kidney has been examined histochemically and electrophoretically and found to exist as isozymes (75). Electrophoretic techniques have also provided evidence that the enzyme exists as isozymes in many other tissues of the mouse such as liver, spleen, intestine, testes, and heart (79). 

Tsukuda, H. (1975). Temperature dependency of the relative activities of liver lactate dehydrogenase isozymes in goldfish acclimated to different temperatures. Comparative Biochemistry and Physiology 52B, 343-345. 

The structure of an enzyme can also vary within a person, since different genes may encode enzymes that catalyse the same reaction. These enzymes are known as isozymes. Isozymes are often specific for different types of tissue. For example, lactate dehydrogenase (LDH) is produced in two forms, the M-type (muscle) and the H-type (heart). The M-type is predominates in tissue subject to anaerobic conditions, such as skeletal muscle and liver tissue, whereas the H-type predominates in tissue under aerobic conditions, such as the heart. Isozymes may be used as a diagnostic aid. For example, the presence of H-type LDH in the blood indicates a heart attack, since heart attacks cause the death of heart muscle with the subsequent release of H-type LDH into the circulatory system. 

Predict its most likely protein translation by submitting the DNA sequence along with the following amino acid sequences of human lactate dehydrogenase isozymes to Procustes analyses. 

Figure 15.10. Structural comparison of the superimposed structures. Structural differences between the superimposed muscle isozyme (3LDH.pdb) and heart isozyme (5LDH.pdb) of lactate dehydrogenase analyzed by the CE server.

Although liver cells can oxidize pyruvate to C02, they contain a predominance of the M4 isozyme, with its low affinity for pyruvate. Lactate enters the liver from the blood plasma and is rapidly converted to pyruvate. 

Figure 7-14. Elution of lactate dehydrogenase (LDH) isozymes with a concave gradient of NADH. Protein, 0.2 mg, in 0.2 ml O.IM sodium phosphate buffer, pH 7.0, ImM /3-mercaptoethanol, and IM NaCl was applied to an AMP analogue-sepharose column (140 X 6 mm, containing 2.5 g wet gel) equilibrated with O.lM sodium phosphate buffer, pH 7.5. The column was washed with 10 ml of the latter buffer, then the isozymes were eluted with a concave gradient of 0.0 to 0.5mM NADH in the same buffer, containing ImM /3-mercaptoethanoI. Fractions of 1 ml were collected at a rate of 3.4ml/hour. The hatched area indicates the pooled fractions that were rechromatographed. [From P. Brodelius and K. Mosbach, FEBS Lett, 35 223 (1973).]...

D-7) Serum LDH (lactate dehydrogenase) eleva- tion. LDH is a widespread intracellular enzyme that acts at this step. Detection of high serum levels is rather nonspecific for localizing the site of damage, but its measurement is helpful in confirming myocardial infarction or injury to the liver, skeletal muscle, or certain other tissues. The presence of 5 different isozymes of LDH helps to further localize the injury, as they are fairly tissue specific. For instance, LDj and LD2 are elevated in myocardial infarction, LD2 and LD3 elevation occur in acute leukemia LDs elevation follows liver or skeletal muscle injury. 

Figure 10.27. Isozymes of Lactate Dehydrogenase. (A) The rat heart LDH isozyme profile changes in the course of development. The H isozyme is represented by squares and the M isozyme by circles. The negative and positive numbers denote the days before and after birth, respectively. (B) LDH isozyme content varies by tissue. [(A) After W.-H. Li, Molecular Evolution (Sinauer, 1997), p. 283 (B) After K. Urich, Comparative Animal Biochemistry (S nngQrVQrlag, 1990), p. 542.]...

Isozymic forms of lactate dehydrogenase in different tissues catalyze the interconversions of pyruvate and lactate. 

---