IDENTIFICATION OF HIGH-MOLECULAR-WEIGHT FORMS OF INOSITOL 1,4,5-TRISPHOSPHATE 3-KINASE IN RAT THYMUS AND HUMAN-LYMPHOCYTES

A soluble inositol 1,4,5-trisphosphate 3-kinase (InsP(3) 3-kinase) has been characterized from extracts of rat thymus. The enzyme was shown to have a molecular weight within the range 98,000-114,000 M(r) as det ermined by regeneration of enzyme activity from sodium dodecyl sulphat e polyacrylamide gels. The enzyme phosphorylates inositol 1,4,5-trisph osphate (InsP(3)) to inositol 1,3,4,5-tetrakisphosphate (InsP(4)) with an apparent K-m of 3.1 +/- 0.4 mu M. The enzyme is stimulated 4-6-fol d by Ca2+/calmodulin and is not recognised by polyclonal antisera rais ed against rat brain InsP(3) 3-kinase A. High levels of InsP(3) 3-kina se activity were also detected in soluble extracts of human lymphocyte preparations. The human lymphocyte enzyme was shown to have a molecul ar weight between 61,000 and 70,000 M(r) as judged by SDS-PAGE, and wa s stimulated approximately 10-fold in the presence of Ca2+/calmodulin. These results establish that InsP(3) 3-kinase from rat thymus and hum an lymphocyte preparations represent high molecular weight isoenzymes of the InsP(3) 3-kinase family.