Hexokinase isoenzymes from anaplastic and differentiated medullary thyroid carcinoma in the rat

The activity, isoenzyme distribution and compartmentation of hexokinase (ADP: D-hexose-6-phosphotransferase, EC 2.7.1.1) were compared in slowly growing, well-differentiated medullary thyroid carcinoma (DMTC) and rapidly proliferating anaplastic thyroid carcinoma (AMTC) in the rat. Individual isoenzymes from either soluble or particulate fractions after solubilization were obtained by fast protein liquid chromatography and were kinetically analyzed either in soluble form or after (re)binding to rat liver mitochondria. These studies were undertaken to test the hypothesis that the growth rate of tumors is correlated with the activity of mitochondrial-bound hexokinase in our tumor system. In contradiction to this hypothesis, we found no difference in either enzyme activity or compartmentation of both kinds of tumors. The major part of enzyme activity was soluble (73 and 78% in DMTC and AMTC respectively). In addition, no major differences were observed in the kinetic properties of the individual isoenzymes of both tumors. Only soluble type II hexokinase from AMTC had a slightly decreased apparent Km for glucose. There appeared to be some differences in isoenzyme composition: both tumors contained type I and type II hexokinase in the soluble as well as in the particulate fractions. However, the proportion was shifted in favor of type II hexokinase in the soluble fraction of AMTC. Additional findings of this study were the following: the affinity of type II hexokinase to both substrates glucose and MgATP2- was significantly less compared to type I hexokinase. However, the inhibition constant for glucose-1,6-diphosphate of both isoenzymes was exactly the same. The bound form of both isoenzymes had the same substrate affinities as the soluble form but was considerably less inhibited by glucose-1,6-diphosphate. In the latter respect, type I and type II hexokinase behaved in the same way.