Hydrolysis phosphate monoesterase

Fig. 1. The major inositol lipids. Phosphatidylinositol (Ptdlns), the major membrane inositol phospholipid, is phosphorylated to phosphatidylinositol 4-phosphate (Ptdlns 4-P) by a phosphatidylinositol kinase (a). Ptdlns 4-P is further phosphorylated to phosphatidylinositol 4,5-bisphosphate (Ptdlns 4,5-P2) by a phosphatidylinositol 4-phosphate kinase (b). Ptdlns 4,5-P, is converted back to Ptdlns 4-P by phosphatidylinositol 4,5-bisphosphate phosphomonoesterase (c) and then to Ptdlns by phosphatidylinositol 4-phosphate monoesterase (d). The pathway of phosphorylation and dephosphorylation constitutes a futile cycle and is only interrupted by agonist-induced hydrolysis of Ptdlns 4,5-P,.

The discovery of a small proportion of a nucleoside containing thymine42 in the ribonucleic acid of two strains of Escherichia coli, in Aerobacter aero-genes, and in commercial, yeast-ribonucleic acid emphasizes the point made previously,26-28 namely, that the nucleic acids may contain constituents other than those heretofore identified. Alkaline hydrolysis of the ribonucleic acid from E. coli gave nucleotides42 (probably the 2- and 3-phosphate esters) which were converted to the nucleoside with prostatic phospho-monoesterase.62 Enzymic hydrolysis of the nucleic acid preparation also led to the nucleoside, which was degraded further to thymine by hydrolysis with perchloric acid.42 There can be little doubt that this carbohydrate derivative of thymine is intimately bound as part of the polynucleotide chain of this particular ribonucleic acid. 

These comparative studies constituted the first example of an enzyme-catalyzed hydrolysis reaction whose stereochemical course was unaffected by sulfur substitution. At the time these experiments were performed, the stereochemical courses of the reactions catalyzed by glycerol kinase (83, 84) and by the bacterial adenylate cyclase (85, 86) had already been compared in the laboratories of Knowles and Gerlt, respectively, and these were also found to be unaffected by the sulfur substitution. A number of other comparisons of this type have been made, and in no case were the stereochemical consequences of the reactions studied with chiral phosphate esters and the chiral thiophosphate analogs found to differ. This agreement suggests that the necessary use of oxygen chiral thiophosphate monoesters to study the stereochemical course of phospho-monoesterases will provide pertinent results for ascertaining whether phosphory-lated intermediates are involved in the reaction mechanism. 

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