The site of nonenzymic glycation of human extracellular-superoxide dismutase in vitro.

The secretory enzyme extracellular-superoxide dismutase (EC-SOD) has affinity for heparin and some other sulfated glycosaminoglycans and is in vivo bound to heparan sulfate proteoglycan. Nonenzymic glycation of EC-SOD, both in vivo and in vitro, is associated with a reduction in heparin affinity, whereas the enzymic activity is not affected. The glycation sites in EC-SOD are further studied in the present article. It is shown that modification of a few of the five lysyl residues of the subunits of the enzyme with trinitrobenzene sulfonic acid nearly abolishes the in vitro glycation susceptibility. From a chymotryptic digest of in vitro glycated EC-SOD, two peptides with affinity for boronate could be isolated. Amino acid sequence analysis showed that both encompassed the carboxyterminal end. epsilon-Glucitol lysine was identified in both peptides at positions 211 and 212. The primary glycation sites in EC-SOD are thus lysine-211 and lysine-212 in the putative heparin-binding domain in the carboxyterminal end.