Hydrolysis of inositol phosphates by plant cell extracts

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Abstract
A gel-filtered soluble fraction prepared from suspension-cultured Nicotiana tabacum cells hydrolysed inositol mono-, bis- and tris-phosphates. At a concentration of 7.5 .mu.M the rates of hydrolysis followed the sequence Ins(1,4,5)P3 > Ins(1,4)P2 > Ins(4)P .simeq. Ins(1)P. The major products of Ins(1,4,5)P3 hydrolysis identified by h.p.l.c. were Ins(1,4)P2 and Ins(4,5)P2. Ins(1,4)P2 was hydrolysed exclusively to Ins(4)P. The inclusion of Ca2+ in the incubation buffer markedly stimulated the hydrolysis of all the inositol phosphate substrates. Under identical conditions, Ca2+ inhibited the hydrolysis of inositol phosphates by soluble extracts prepared from rat brain. Half-maximal stimulation of Ins(1,4)P2 hydrolysis was obtained at free [Ca2+] of 0.6 and 1.2 .mu.M when the Mg2+ concentration in the incubations was 0.3 and 1.0 mM respectively. This effect of Ca2+ was exerted solely by increasing the Vmax. Of hydrolysis without affecting the Km for Ins(1,4)P2. Again, in contrast with brain, the hydrolysis of inositol bis- or mono-phosphates was insensitive to high concentrations of Li+. We conclude that plants contain specific Li+-insensitive inositol phosphate phosphatases that are regulated by low concentrations of Ca2+ in a manner which is different from that observed in mammalian tissues.