IN-VITRO METABOLISM OF INOSITOL 1,4,5-TRISPHOSPHATE BY NEUROSPORA-CRASSA

The in vitro metabolism of Ins(1,4,5)P-3 by a soluble and a particulat e fraction of Neurospora crassa revealed the presence of an Ins(1,4,5) P-3 kinase activity which was associated with the soluble fraction of this organism but was not detected in the particulate fraction. The pr oduct was tentatively identified as Ins(1,3,4,5)P-4 by co-chromatograp hy with an authentic standard. A second minor unidentified InsP(4) was also detected. The kinase was not stimulated by Ca2+/calmodulin and w as unaffected by EGTA. Phosphomonoesterase activity was predominantly associated with the particulate fraction producing Ins(1,4)P-2, Ins(1, 5)P-2 and Ins(4,5)P-2 indicating the presence of 1', 4' and 5'-phospho monoesterases. N. crassa therefore differs from other classes of eukar yotes where Ins(1,4,5)P-3 metabolism has been studied in possessing a specific 4'-phosphomonoesterase. In the particulate fraction, 1'-phosp homonoesterase was the principal route for Ins(1,4,5)P-2 degradation w hereas all three phosphomonoesterases were equally active in the solub le fraction. All phosphomonoesterase activity was Mg2+-dependent. Furt her degradation to inositol monophosphates was not observed under the conditions used in this study.