Lactate dehydrogenase isozymes and their relationship to lens cell differentiation

Changes in the activity of lactate dehydrogenase (LDH) (l-lactate: NAD⁺ oxidoreductase EC 1.1.1.27) isozymes are associated with the growth and differentiation of bovine lens cells. Calf and adult lens epithelial cells contain all 5 isozymes. The cathodal forms are most active in the calf-epithelial cells; the anodal forms are most active in the fiber cells. This transition from cathodal to anodal forms of lactate dehydrogenase in the epithelial cells is associated with cellular aging. During the differentiation of an epithelial cell to a fiber cell, in calf and adult lenses there is an enhancement of the transition from cathodal forms to anodal forms. The regulation of lactate dehydrogenase subunit synthesis may be associated, therefore, with the replicative activity of these cells. In cells having the greatest replicative activity (calf epithelial cells) the cathodal isozymes are most active; in cells having a decreased mitotic activity (adult epithelial cells) the anodal isozymes are most active. The non-replicative fiber cell of calf and adult shows a further transtion toward the anodal forms. Although lens fiber cells have a low rate of oxidative metabolism lactate dehydrogenase-I is the most active isozyme in these cells. Kinetically, lactate dehydrogenase-I factors other than, or in addition to, the regulation of carbohydrate metabolism are involved in regulating the synthesis of lactate dehydrogenase subunits.