Abstract
An unusually heat-labile fraction of glutathione reductase appears in human lens at about an age of 32 years. No such change was found in glucose 6-phosphate dehydrogenase nor in 3-phosphoglycerate kinase. The change is an intrinsic property of glutathione reductase. A greater proportion of the altered glutathione reductase was found in the core, the older part, of the lens. No evidence of a second band of activity was obtained after electrophoresis. Possible interpretations of the results, including errors of protein synthesis, production of a new isoenzyme and post-synthetic changes, are discussed.