Characterization of a calcium-modulated adenylate cyclase from abalone spermatozoa.
Abalone spermatozoa contain a particulate adenylate cyclase that displays maximal catalytic activity when Mn2+ is present as a metal cofactor in excess of ATP. Unlike other sperm adenylate cyclases, the abalone enzyme displays a high Mg2+-supported catalytic activity (Mg2+/Mn2+ activity ratio = 0.8). Kinetics analyses demonstrate that the enzyme contains both a MgATP catalytic site and a separate Mg2+ regulatory site. Mg2+-supported enzyme activity, however, is not stimulated by guanine nucleotides, NaF, cholera toxin, forskolin, or a variety of hormones. The enzyme from unfractionated sperm homogenates is inhibited by added Ca2+ in a concentration-dependent manner, when EGTA is not present in the assay. Methylxanthines, such as 1-methyl-3-isobutylxanthine and theophylline, also inhibit enzyme activity in a concentration-dependent manner through a noncompetitive mechanism. On the other hand, when intact cells are preincubated with Ca2+ prior to breakage and assayed for enzyme activity, Ca2+ stimulates enzyme activity at low concentrations. Enzyme activity of intact sperm preincubated with methylxanthines, in either the absence or presence of added Ca2+, is also stimulated. This effect is expressed via an effect on the velocity of the enzyme. A-23187 has similar stimulatory effects on the enzyme under these conditions. These data provide further support for the role of Ca2+ conductance in modulating sperm adenylate cyclase activity. The abalone sperm enzyme also appears to have regulatory properties that are unique among other sperm types.
G S
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Kopf
V D
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Vacquier
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000067956:Adenylyl Cyclase Inhibitors; D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002413:Cations, Divalent; D002474:Cell-Free System; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation / Q000187:drug effects; D007700:Kinetics; D008297:Male; D008974:Mollusca / Q000502:physiology*; D013094:Spermatozoa / Q000201:enzymology*; D013806:Theophylline / Q000494:pharmacology
D000067956:Adenylyl Cyclase Inhibitors; D002413:Cations, Divalent; D000001:Calcimycin; D013806:Theophylline; D000262:Adenylyl Cyclases; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine