Studies on the Crystalline Lens : Viii. A Synthetic Medium for Lens Culture and the Effects of Various Constituents on Cell Division in the Epithelium

Abstract

is known concerning the energetics of the lens. Although this organ does not have the specialized energy requirements of muscle or liver, it does require energy for active transfer of metabolites across the capsule and cell membranes, for the continual turnover of its structural elements, and for various other metabolic processes. Most of the energy for metabolic processes in other tissues is now thought to be made available as phosphate. High-energy phosphate is energy contained in a particular type of unstable chemical bond involving phosphorus. The energy is transported from one metabolite to another chiefly by adenosinetriphosphate (ATP) and phosphocreatine. Thus, quantities of these substances present in a tissue would seem to give an indication of the energy available for cellular function. (A highly schematic diagram showing the relation of the production, utilization, and storage of high-energy phosphate is shown in Figure 1.) High-energy phosphate is probably formed in the lens, as elsewhere, through the phosphorylation of adenosinediphosphate (ADP) by energy furnished from the breakdown of glucose to lactic acid (anaerobic glycolysis) and from oxidation of pyruvic acid and other metabolites (respiration). Recent work by Christiansen 1 indicates that in the lens glucose is converted to lactic acid by means of anaerobic glycolysis. This process yields 2 moles of high-energy phosphate per mole of glucose utilized. The lens has been shown to consume approximately 3 mg. (0.015 mM) of glucose per day,2 and since the energy derived from anaerobic glycolysis is not sufficient to satisfy energy requirements for even the replacement of protein,3 addi¬ tional energy presumably must be supplied by respiratory processes. These proc¬ esses usually involve the tricarboxylic-acid cycle and produce approximately 38 moles of high-energy phosphate for each mole of glucose oxidized. Aerobic metab