Soluble, Insoluble and Latent Hexokinases in the Mammalian Lens

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Abstract
The mammalian lens contains two distinct isozymes of hexokinase, classical type I and type II. These isozymes are present in the soluble and insoluble fraction of a lens homogenate. The capsule is devoid of hexokinase activity. 35-80% of all the hexokinase in the epithelium is bound to insoluble material sedimenting at 96,600g. Practically all of the bound enzyme is the type I isozyme. A large part of the particulate form of hexokinase is inactive (or latent) unless solubilized by a detergent (Triton X-100) or other means. In the cortex and nucleus, 95% of all the hexokinase is soluble and 70-94% of this is the type II isozyme. The activity of hexokinase is greatest in the epithelium; there is a steady drop in specific activity in the deeper layers of the lens and in the nucleus there is essentially no measurable activity. Human lenses obtained postmortem or senile cataracts obtained at surgery appeared to be devoid of type II hexokinase. Human lens epithelium maintained in tissue culture was studied with starch gel electrophoresis. Both types I and II hexokinase were shown to be present. The significance of the loss of the type II isozyme in senile cataracts and postmortem specimens remains to be determined. At 37.5 °C in a glucose-free environment, type II hexokinase in an enzyme preparation and in the intact lens is very unstable. This has been a consistent observation with all the lenses studied. Whether this is a significant site of metabolic vulnerability in the lens is at present unknown.

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