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GUANYLATE CYCLASES IN CNS: ENZYMATIC CHARACTERISTICS OF SOLUBLE AND PARTICULATE ENZYMES FROM MOUSE CEREBELLUM AND RETINA

Guanylate cyclase activity is present in both soluble and particulate fractions of homogenates of mouse cerebellum and retina. Soluble guanylate cyclases in cerebellum and retina have an apparent Km for GTP of approx 40 and 70 μM, respectively; are stimulated by Ca2+ and Mg2+ in the presence of low Mn2+; and do not respond to NaN3, NH2OH or detergent. The particulate guanylate cyclase found in brain has an apparent Km GTP of 237 7mu;M, is not stimulated by Ca2+ or Mg2+ in the presence of low Mn2+, but is stimulated by NaN3, NH2OH, and detergent. In particulate fractions of normal retina, guanylate cyclase has two apparent Km GTP values (42 and 225 μM); has higher activity at low concentrations of Mn2+ (0.5 mM) than at high concentrations (5.0 mM); is inhibited by Ca2+; and does not respond to NaN3, NH2OH, or detergent. Retinas essentially devoid of photoreceptor cells (from mice with photoreceptor dystrophy) have soluble guanylate cyclase activity which is similar to that in normal retina, but have only 4% as much particulate guanylate cyclase activity. This residual particulate guanylate cyclase has an apparent Km GTP value of 392 μM and other properties similar to particulate guanylate cyclase from brain. These data indicate the presence of three distinguishable guanylate cyclases in CNS: (1) a soluble enzyme present in both brain and retina: (2) a particulate enzyme which is also present in brain and in the inner or neural retina: and (3) another particulate enzyme which is apparently unique and confined to retinal photoreceptor cells.