ASCORBIC ACID GLYCATION & SENILE CATARACT FORMATION

点击翻译按钮获取中文摘要

The non-enzymatic glycosylation of proteins represents a slow modification reaction which proceeds continuously in serum and in tissues. This reaction is termed the Maillard reaction and is receiving increasing attention as it may be responsible for many of the sequelae observed during diabetes. It is known that the initial carbohydrate-protein adducts undergo a complex series of chemical reactions, which can ultimately cause the production of 1) protein-protein crosslinks, 2) a variety of protein-bound chromophores and 3) complex molecules which produce a blue fluorescence when irradiated with ultraviolet light. These secondary reactions are likely to be very important in lens tissue, because lens exhibits little or no protein turnover, and proteins in the lens nucleus may have been present throughout life. Since it is known that senile cataractous lenses do contain crosslinked proteins as well as protein-bound chromophores and fluorophores similar to those produced by glycosylation, it has been suggested that the products of the Maillard reaction may be a major factor in cataractogenesis. This reaction was thought to be limited to glucose, however, it has been recently shown that ascorbic acid is also capable of modifying lens proteins via the Maillard reaction. Ascorbic acid is present in higher concentrations and is more reactive with lens proteins than glucose, yet little is known about the chemistry of ascorbic acid protein adducts. The work described here will be undertaken to isolate and identify the products formed between ascorbic acid and model alpha-N blocked amino acids. These results will be compared to the products produced by the in vitro modification of lens crystallins by ascorbic acid and to products isolated from human lens proteins. The specific sites of modification in alpha-crystallin will be identified, and in addition the effects of these modifications on the properties of alpha-crystallin will be investigated. Preliminary experiments will be carried out to describe the nature of the protein crosslinks induced by incubating lens crystallins with ascorbic acid under physiological conditions.