[Quantitative histochemical study of the reaction kinetics to inosine-5-monophosphate dehydrogenase and 5'-nucleotidase in cryostat sections of rat liver].

: The data are presented on kinetics of histochemical enzymatic reactions demonstrating two enzymes taking part in the purine metabolism--inosine-5-monophosphate dehydrogenase (IMPD) and 5'-nucleotidase (5'-NT). It is shown that IMPD has a very weak affinity to substrate IMP (KM = 2.5 . 10(-2) M); this fact partially explains the low rate course in cryostat sections. Cytosol and membrane forms of 5'-NT have the maximum affinity to AMP (KM for membrane and cytosol forms being 1 . 10(-3) M and 2.1 . 10(-3) M, resp.). When IMP is used as a substrate, 5'-NT localized in cytosol has much lower KM as compared to the membrane form. Cytoplasmic 5'-NT is thermostable. It is suggested that a low rate of histochemical reaction demonstrating IMPD is caused by a degradation of substrate by the membrane form of 5'-NT, the Michaelis constant of which is 5 times less than that of IMPD.