TISSUE DISTRIBUTION AND INTRACELLULAR-LOCALIZATION OF THE 75-KDA INOSITOL POLYPHOSPHATE 5-PHOSPHATASE

The 75-kDa inositol polyphosphate 5-phosphatase (75-kDa 5-phosphatase) hydrolyses several important mediators of intracellular calcium homeo stasis, including inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3], inosit ol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P-4] and phosphatidylinosito l 4,5-bisphosphate [PtdIns(4,5)P-2]. Northern analysis of various huma n tissues revealed the 75-kDa 5-phosphatase has a ubiquitous expressio n, where differential splicing may occur in specific tissues. Prominen t expression of a 4.4-kb transcript was noted in human lung, thymus, t estes and placenta, and a 4.6-kb transcript was observed in heart, bra in, kidney, ovary and colon. Determination of the intracellular locati on of the enzyme by indirect immunofluorescence, demonstrated that the 75-kDa 5-phosphatase was associated with mitochondrial and cytosolic cellular compartments. Immunoprecipitation of the total cell homogenat e of human lung carcinoma cells (A549) with anti-(recombinant 75-kDa 5 -phosphatase) antibodies revealed that the 75-kDa 5-phosphatase is the major PtdIns(4,5)P-2 5-phosphatase in this cell line. Analysis of Ptd Ins(4,5)P-2 5-phos phatase activity in subcellular fractions of A549 c ells revealed peak 75-kDa 5-phosphatase enzyme activity in the cytosol ic and mitochondrial enriched fractions. Immunoblot analysis further c onfirmed the mitochondrial location of the enzyme. This study demonstr ates the tissue distribution and intracellular location of the 75-kDa 5-phosphatase and reveals a novel location for an enzyme involved in p hosphatidylinositol turnover.