[Effect of freezing-thawing on the activity of lactate dehydrogenase isoenzymes].

The activity of isolated M4- and H4-lactate dehydrogenase isoenzymes is studied after freezing in the saline solution to -8, -9.5, -23 and -30 degrees C. It is shown that the activity of H4-lactate dehydrogenase does not change, M4-lactate dehydrogenase being partially inactivated. It is demonstrated that a reduction in the M4-LDH activity is due to the effect of high salt concentrations on the freezing-out water from the solution and is an irreversible process. The fluorescence spectra of these isoenzymes are studied after freezing-thawing, and the interpretation of the observed changes in these spectra is suggested on the basis of possible alterations in the quaternary structure and sonformation of the enzyme molecule subunits. It is shown that the substrates and coenzymes of lactate dehydrogenase (NAD, NADH, pyruvate, lactate) completely or partially prevent the inactivation of M4-isoenzymes at freezing-thawing.