Skeletal muscle lactic dehydrogenase

Both are abundant in skeletal muscle, myocardium, liver, and erythrocytes, so that hemolysis must be avoided, and in serum they may be assayed spectrophotometrically by their conversion of phosphate-buffered pyruvate to lactate (R6, W16) or oxalacetate to malate (S25) at the expense of added NADH2, when the rate of decrease of optical density at 340 m x thus measmes the serum activities of the respective enzymes. Recently, however, the reverse reaction has been found best for serum lactic dehydrogenase assay (A2a). In conventional spectrophotometric units the normal ranges are 100-600 units per ml for lactic dehydrogenase (W16) and 42-195 xmits per ml for malic dehydrogenase (S25) as before, one conventional spectrophotometric unit per ml = 0.48 pmoles/ minute/liter (W13). 

Lactic dehydrogenase (LDH) is a tetramer composed of two kinds of subunits called M and H. A large fraction of the skeletal muscle LDH is MMMM. A large fraction of the heart LDH is HHHH. Other tissues contain hybrid isozymes containing both M and H subunits. How many different LDH isozymes are possible Assume that the subunits are arranged in a square or tetrahedral" fashion so that there is no sequence" (i.e., HMMM is the same as MMMH). 

Figure 3 Far-UV circular dichroism spectra of native, denatured, and aggregated forms of LDH-M4 (tetrameric porcine lactic dehydrogenase from skeletal muscle). ( ) native and (O) renatured LDH-M4 at a concentration of 1.5 mg/ml. ( ) LDH-M4 denatured at pH 2 or at ( ) 6 M guanidine hydrochloride, same concentration (A) aggregated LDH-M4 dispersed by low-energy sonication at a concentration of 0.05 mg/ml (determined gravimetrically from the dry weight at 105 C). From Zettlemeissl et al, (75) reproduced with permission.

Lactic dehydrogenase E>ecrease liver, kidney, brain, heart, and skeletal muscle (BID >... 

This inhibitor can even discriminate between isoenzymes. Thus, whereas it irreversibly inhibits the lactic dehydrogenase of skeletal muscle, it does not affect that of the heart. A related compound, 5-(phenoxycarbonylamino)salicylic acid, irreverisbly inhibits the heart (but not the skeletal) isoenzyme, whereas its 4-isomer irreversibly inhibits the skeletal (but not the heart) isoenzyme (Baker and Patel, 1964 Baker, 1967). 

Conway, E. J., 1963, Significance of various factors including lactic dehydrogenase on the active transport of sodium ions in skeletal muscle. Nature 198 760. 

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