The homocysteine thiolactonyl derivative, thioretinaco ozonide, is believed to function as an electron acceptor in oxygen metabolism and as the binding site for adenosine triphosphate (ATP) synthesis by mitochondria, preventing damage by free radical oxidants in resting cells. During cell division, methionine is converted to homocysteine thiolactone, converting thioretinaco to thioco, increasing free radical oxidants, and oxidizing cellular glutathione and ascorbate. Homocysteic acid has growth hormone activity and releases insulin-like growth factor in hypophysectomized rats, promoting oxidation of homocysteine thiolactone to sulfated glycosaminoglycans of cartilage. The free base of homocysteine thiolactone produces keratinization, squamous metaplasia, dysplasia, and carcinogenesis in normal mouse tissues. The efficiency of homocysteine thiolactone metabolism declines with aging, explaining decreased formation of adenosyl methionine in aging and suggesting loss of thioretinaco ozonide from membranes of aging cells. The effects of aging on enzyme activity, connective tissues, lipid synthesis, auto-immune diseases, atherogenesis and carcinogenesis are related to these changes in homocysteine metabolism.