AN ISOENZYME OF ACID ALPHA-GLUCOSIDASE WITH REDUCED CATALYTIC ACTIVITY FOR GLYCOGEN

Abstract

Both the common and a variant isozyme of acid .alpha.-glucosidase were purified from a heterozygous [human] placenta with CM-Sephadex, ammonium sulfate precipitation, dialysis, Amicon filtration, affinity chromatography by Sephadex G-100 and DEAE-cellulose chromatography. Three and 2 activity peaks, from the common and variant isozymes, respectively, were obtained by DEAE-cellulose chromatography using a linear NaCl gradient. The 3 peaks of activity of the common isozyme were eluted with 0.08, 0.12 and 0.17 M NaCl, whereas the 2 peaks of the variant, with 0.01 and 0.06 M NaCl. The pH optimum and thermal denaturation at 57.degree. C were the same in all enzyme peaks of both isozymes. Rabbit anti-acid .alpha.-glucosidase antibodies produced against the common isozyme cross-reacted with both peaks of the variant isozyme. The 2 isozymes shared antigenic identity and had similar Km''s with maltose as substrate. Normal substrate saturation kinetics were observed with the common isozyme when glycogen was the substrate, but the variant produced an S-shaped saturation curve indicating a phase of negative and positive cooperativity at low and high glycogen concentrations, respectively. The activity of the variant was only 8.6% and 19.2% of the common isozyme when assayed with nonsaturating and saturating concentrations of glycogen, respectively. A similar rate of hydrolysis of isomaltose by both isozymes was found indicating that the reduced catalytic activity of the variant isozyme toward glycogen is not the result of a reduced ability of this enzyme to cleave the .alpha.-1 6 linkages of glycogen.