The human inositol phosphate multikinase (IPMK, 5-kinase) has a preferred 5-kinase activity over 3-kinase and 6-kinase activities and a Substrate preference for inositol 1,3,4,6-tetrakisphosphate (Ins(1 3,4,6)P-4) over inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3) and inositol 1,3,4,5-tetrakisphosphate (Ins(l,3,4,5)P-4). We now report that the recombinant human protein can catalyze the conversion of inositol ,4,5,6-tetrakisphosphate (Ins(1,4,5,6)P-4) to Ins(1,3,4,5,6)P-5 in vitro; the reaction product was identified by HPLC to be Ins(1,3,4,5,6)P-5. The apparent V-max was 42 nmol of Ins(1 3,4,5,6)P-5 formed/inin/ing protein, and the apparent K-m was 222 nM using Ins(1,3,4,6)P4 as a substrate; the catalytic efficiency was similar to that for Ins(1,4,5)P-3. Stable over-expression of the human protein in HEK-293 cells abrogates the in vivo elevation of Ins(1,4,5,6)P4 from the Salmonella dublin SopB protein. Hence, the human 5-kinase may also regulate the level of Ins(1,4,5,6)P-4 and have an effect on chloride channel regulation. (c) 2005 Elsevier Inc. All rights reserved.