Purification and characterization of particulate guanylate cyclase from sea urchin spermatozoa

The particulate form of guanylate cyclase from sea urchin spermatozoa was purified to apparent homogeneity by chromatography on GTP-Sepharose and DEAE-Sepharose and by preparative gel electrophoresis. The sedimentation coefficient (S20,w) was 6.8 and the Stokes radius was 5.1 nm, from which a native molecular weight of 157,000 was calculated. A single protein or periodic acid-Schiff staining band of 135,000 Da was observed after Na dodecyl SO4 gel electrophoresis. Antibody was produced to guanylate cyclase and was shown by electrophoretic transfer experiments (Western blot) to interact with only the Mr = 135,000 band in cases where all of the detergent-extracted protein from spermatozoa was added to the Na dodecyl SO4 gels. Although guanylate cyclase was normally bound to concanavalin A-Sepharose, after endoglycosidase H treatment it failed to bind. Treatment of the enzyme with endoglycosidase H did not alter guanylate cyclase activity, but the apparent size of the enzyme decreased to 72,000 Da on Na dodecyl SO4 gels. An analysis of carbohydrate composition indicated that the oligosaccharides contained N-acetylglucosamine, mannose, galactose, and 2-aminoerythritol in molar ratios (1:3:0.75:2); after endoglycosidase H treatment the enzyme contained essentially no carbohydrate. Major amino acids in the enzyme were aspartic (Asn) and glutamic (Gln) which accounted for approximately 25 mol % of the enzyme amino acid composition. The purified enzyme displayed linear kinetics on double reciprocal plots and had a KMnGTP = 133 microM, KM2+ = 138 microM, KiMnGTP = 122 microM, KiMn2+ = 127 microM, and a V max in excess of 15 mumol of cyclic GMP formed/min/mg of protein at 30 degrees C. Sodium nitroprusside did not stimulate the enzyme in either the presence or absence of added hemeproteins. These results indicate that the particulate form of guanylate cyclase from sea urchin spermatozoa is a glycoprotein which is distinctly different than the soluble form of the enzyme found in mammalian tissues.