Distinguishing the type I and type II isozymes of hexokinase: the need for a reexamination of past practice.

The type I and type II isozymes of hexokinase coexist in insulin-sensitive tissues, such as cardiac and skeletal muscle and adipose tissue. Based on an early report that the purified type I isozyme was stable at 45 degrees C whereas the purified type II isozyme was not, investigators in a number of studies have used heat lability as a criterion for distinguishing these isozymes in crude tissue extracts or subcellular fractions; that is, activity lost after incubation at 45 degrees C was believed to be type II while remaining activity was considered type I. This extrapolation is dangerous because thermal lability can be markedly affected by the solvent environment, including the presence or absence of other proteins. In the present study, the rate of thermal inactivation of the type I isozyme has been shown to vary by at least an order of magnitude in soluble and particulate fractions prepared from rat heart and brain. Thus, the use of thermal stability as a general criterion for identifying the type I isozyme is invalid, and conclusions based on thermal inactivation as a means for distinguishing the type I and type II isozymes need to be reconsidered.