Net proteolysis in glucose-deprived rat lenses incubated in amino acid-free medium

Abstract

A study of proteolysis was undertaken in rat lenses cultured in balanced salt solution, without amino acids. When glucose was present, slow loss of amino acids to the medium occurred, but sodium and potassium levels remained normal for at least 48 hr. Lenticular glutathione slowly disappeared, with matching increases of glycine and cysteine in the lens plus medium. Lenticular ATP gradually decreased, with matching increase of free phosphate. Ascorbate decreased steadily during incubation, and lactate loss by diffusion exceeded replacement via glycolysis. Lens taurine decreased steadily, but the total in lens plus medium remained constant. In the absence of glucose, the initial loss of ATP and glutathione were faster and some exchange of sodium and potassium occurred. Starting abruptly after 24 hr, when ATP was depleted, rapid net proteolysis was observed. The quantities of proteogenic amino acids in lens plus medium increased linearly to several times the initial values by 48 hr. Increased lens levels of proteogenic amino acids were accompanied by marked influx of sodium and water. Calcium also increased in the lens. Loss of lens taurine was accelerated and depletion of lactate was faster. The results obtained suggest that disappearance of ATP in glucose-deprived lenses is followed by onset of net proteolysis.