Hexokinase of calf trabecular meshwork

Abstract

Although aqueous outflow is most likely a passive, nonenergy-dependent process, alterations in cellular function in the trabecular meshwork presumably are involved in the development of some types of glaucoma. Accordingly, it seems important to define both the normal and abnormal biochemistry of this tissue. The authors have chosen glycolysis as their starting point, concentrating on the regulatory enzymes, hexokinase, and, in a companion paper, phosphofructokinase. Hexokinase activity has been measured in the 100,000 X g supernatant of homogenates prepared from excised calf trabecular meshwork. Treatment of the homogenate with Triton X-100 before centrifuging caused a twofold increase in measurable activity. Electrophoresis on cellulose acetate revealed types I and II isoenzymes. Electrophoresis on starch gel further resolved type 1 into the adult and fetal subtypes. The principal isoenzyme type released into solution by Triton X-100 was type 1. The kinetic behavior of hexokinase was measured by varying the concentrations of glucose at saturating levels of ATP. Kms calculated from these plots were 7.15 X 10(-2) M, 1.78 X 10(-3) M, and 1.19 X 10(-4) M. Apart from the fetal form of type I hexokinase, the isoenzymes from trabecular meshwork resemble those of other ocular tissues and most extraocular tissues. The special role, if any, of the fetal isoenzyme in regulating glycolysis is not known. Possibly, it is a "kinetically adaptable" isoenzyme.