Some properties of the phosphoinositide inositolphosphohydrolase of guinea-pig intestinal mucosa supernatant fraction are described. The enzyme has an absolute requirement for metal ions, the most efficient activator being CaZf. The pH optimum is 5.3 in acetate buffer and 5.9 in maleate; activity is higher in the latter. The effects of detergents and various reagents are described. Corn phosphatidyl inositol is hydrolysed faster than brain phosphatidyl inositol ; the fatty acid compositions of these lipids have been compared. The enzyme is remarkably specific for the phosphoinositides. The composition of the enzyme preparation has been studied. The enzyme appears to be widely distributed in animal tissues.Phospholipase C activity appears to play a significant part in the inetabolism of the phosphoinositides. Rodnight [i] showed that brain preparations from several animal species catalysed the release of inositol phosphate from the inositol phospholipid fraction of brain. Thompson and Dawson [2--41 later succeeded in separating polyphosphoinositide phosphomonoesterase and phosphodiesterase (phospholipase C) from dialysed extracts of ox-brain. Dawson[5] had previously described a preparation from ox pancreas which released phosphate from phosphatidyl inositol. Kemp, Hiibscher and Hawthorne [S] obtained an enzyme preparation from rat liver which hydrolysed phosphatidyl inositol releasing both glycerylphosphorylinositol and inositol phosphate. This preparation also released inositol diand triphosphates from the polyphosphoinositides [7].The hydrolysis of inositol lipids by preparations from intestinal mucosa is now reported and the properties of an active phospholipase C from the supernatant fraction are described. Some of the results have been reported briefly [S]. Very recently Thompson [9] has described a similar enzyme in the supernatant fraction of brain. The reaction is as follows :The rapid turnover of the phosphate moieties in the phosphoinositides of many tissues suggest an important physiological function, possibly in relation to transport processes. Detailed information about t,he enzymes concerned in this turnover should help in the elucidation of function. Much of the earlier work is of limited value because the relevant phospholipase was not assayed under zero-order conditions + 1,2-diglyceride or because other enzymes attacking the same substrate were present. There was also relatively little information about subcellular distribution. The guinea-pigs used in this work were albinos fed on a mixture in equal parts of Spillers' (London) intensive layers pellets and Hill's (Poole) R. G. P. pellets given ad libitum, water ad libitum, about 20 g cabbage daily and limited amounts of hay. Females were normally used (weight 200-400 g) although enzyme levels were equally high in males. Protein was estimated by a biuret method based on that of Weichselbaum (131. Samples were first precipitated by adding an equal volume of 100/,

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1. The purification of creatine kinase (ATP-creatine phosphotransferase, EC 2.7.3.2) from ox brain by a method that is quicker, simpler and gives much higher yields than other published procedures is described. 2. Stoicheiometric inhibition studies with iodoacetate showed that the enzyme, like that from muscle, has two reactive thiol groups that are essential for enzyme activity. 3. The amino acid sequence around the essential thiol groups was determined and found to be virtually identical with that in creatine kinases from rabbit and ox muscle, and very similar to that found in arginine kinase; the evolutionary significance of this is discussed. 4. The identification of DNS-amino acids on thin layers of silica gel was found to have, in many cases, distinct advantages over that on polyamide layers.

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