The triphosphoinositide phosphomonoesterase of brain tissue

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Abstract
Triphosphoinositide phosphomonoesterase was isolated from ox-brain extracts by ammonium sulphate fractionation and gradient-density zone electrophoresis. It was slightly contaminated with phosphodiesterase. Triphosphoinositide and, to a lesser extent inositol triphosphate were dephosphorylated. The enzyme had a pH optimum of about 6[center dot]8 and required Mg2+ (or Mn2+) ions as an essential cofactor. Part of the requirement for Mg2+ions was replaceable by a component of the pH 5[center dot]0 supernatant of ox-brain extract and also by substances that would decrease the excess of negative charge on the substrate molecule. Sodium dodecly sulphate antagonized the effect of all activators except for the ox-brain pH 5[center dot]0 supernatant factor. Sodium chloride stimulated the reaction when suboptimum amounts of certain of the substances that replace part of the requirement for Mg2+ions were present. The enzyme required reduced glutathione or cysteine for full activity. p-Chloromercuribenzoate, Hg2+ ions and phenylmercury acetate were inhibitory in low concentrations. Iodoacetate, iodoacetamide and N-ethylmaleimide did not inhibit.