Covalent glycoinositolphospholipid (GPI) binding to hemoglobin is associated with insulin-activation of erythrocyte membrane protease

Abstract

Recently, it was demonstrated that prolonged hyperinsulinism associated with hypoglycemia, both in vivo and in vitro, caused covalent glycoinositolphospholipid (GPI) binding to the C termini of both hemoglobin beta-chains, which resulted in the formation of a novel, hitherto unrecognized, minor hemoglobin fraction (GPI-Hb) (Niketic et al. Biochem. Biophys. Res. Commun. 239 (1997)435). In this study, it was demonstrated that exposure of erythrocyte membranes to insulin causes the activation of membrane protease as well as that the formation of GPI-Hb parallels its activity. It is suggested that the insulin-activated protease is able to catalyze. albeit slowly, the transpeptidation, i.e., the replacement of the carboxy-terminal amino acid(s) residues of the Hb beta-chains with GPI as an exogenous nucleophile. To our knowledge the present results show for the first time that insulin stimulates protease activity in erythrocyte membranes, as well as that insulin-activated protease may be involved in post-translational GPI binding to proteins.

U našim ranijim radovima pokazano je da u uslovima hiperinsulinizma i hipoglikemije, in vivo i in vitro, dolazi do kovalentnog vezivanja glikoinozitolfosfolipida (GPI) za karboksilne krajeve oba β-niza molekula hemoglobina (Hb), što se manifestuje nastajanjem nove, do tada nepoznate, manje frakcije hemoglobina (GPI-Hb) (Niketić et al., Biochem. Biophys. Res. Commun. 239 (1997) 435). U ovom radu je pokazano da vezivanje insulina za membrane eritrocita izaziva aktiviranje membranske proteaze, te da je nastajanje GPI-Hb u korelaciji sa proteaznom aktivnošću. Pretpostavljeno je da proteaza aktivirana insulinom može, mada sporo, da katalizuje reakciju transpeptidacije, tj. zamenu aminokiselinskih ostataka sa karboksilnog kraja β-nizova molekula Hb sa GPI-lipidom kao egzogenim nukleofilom. Prema našem saznanju opisani rezultati prvi puta pokazuju da insulin stimuliše proteaznu aktivnost u eritrocitima, te da je ova aktivnost povezana sa post-translacionim vezivanjem GPI-lipida za proteine.