Molecular basis for the accumulation of acidic isozymes of triosephosphate isomerase on aging.

Triosephosphate isomerase exhibits acidic electrophoretic subforms in many tissues and these isozymes appear to increase during aging of erythrocytes and the eye lens. Incubation of the pure enzyme under mild alkaline conditions results in the generation of acidic forms which are identical to those found in vivo. Structural analysis of these isozymes from both in vivo and in vitro studies showed that they are the result of deamidation of two specific asparagines (Asn-15 and Asn-71). These labile asparagines are located in the subunit-subunit contact sites, and the deamidations introduce a total of four new negative charges in the contact site. The positions of the new aspartic acid residues are juxtaposed, thus creating charge-charge interactions which cause the dimeric enzyme to dissociate more readily. These studies (1) explain the molecular basis for the acidic isozymes observed in many tissues, (2) show that the deamination process is spontaneous and requires no intrinsic cell factors, (3) show that the deamination occurs in a sequential fashion with the deamidation of Asn-71 preceding the deamidation of Asn-15, and (4) suggest that proteolytic degradation processes may become altered during aging resulting in the accumulation of the deamidated intermediates of the normal catabolic process.