Substantial amounts of three [3H]InsP5 isomers were detected in [3H]inositol-labelled human lymphoblastoid (T5-1) cells. Their structures were determined by h.p.l.c. [Phillippy & Bland (1988) Anal. Biochem. 175, 162-166], and by utilizing a stereospecific D-inositol 1,2,4,5,6-pentakisphosphate 3-kinase from Dictyostelium discoideum [Stephens & Irvine (1990) Nature (London) 346, 580-583]. The structures were: inositol 1,3,4,5,6-pentakisphosphate, D-inositol 1,2,4,5,6-pentakisphosphate and L-inositol 1,2,4,5,6-pentakisphosphate. The relative proportions of these isomers (approx. 73:14:14 respectively) were unaffected by cross-linking anti-IgD receptors. The T5-1 cells also contained InsP6 and three Ins P4s, which were identified as the 1,3,4,5, 1,3,4,6 and 3,4,5,6 isomers. In incubations with permeabilized T5-1 cells, both 1,3,4,6 and 3,4,5,6 isomers of InsP4 were phosphorylated solely to Ins(1,3,4,5,6)P5. Permeabilized cells also dephosphorylated InsP6, even in the presence of a large excess of glucose 6-phosphate to saturate non-specific phosphatases. In the latter experiments the following isomers of InsP5 accumulated: D- and/or L-Ins(1,2,3,4,5)P5, plus D- and/or L-Ins(1,2,4,5,6)P5. This demonstration that multiple isomers of InsP5 may be formed in vivo and in vitro by a transformed lymphocyte cell line adds a new level of complexity to the study of inositol polyphosphate metabolism and function.
Tri-methylation of Histone H3 Lysine 4 Facilitates Gene Expression in Ageing Cells
