Metabolic evidence for the order of addition of individual phosphate esters to the myo-inositol moiety of inositol hexakisphosphate in the duckweed Spirodela polyrhiza L.

The aquatic monocotyledonous plant Spirodela polyrhiza was labelled with [P-32]P-i for short periods under non-equilibrium conditions. An InsP(6) fraction was obtained and dissected by using enantiospecific (enzymic) and non-enantiospecific (chemical) means to determine the relative labelling of individual phosphate substituents on the inositol ring of InsP(6). Phosphates in positions D-1, -2, -3, -4, -5 and -6 contained approx. 21%, 32-39%, 9-10%, 14-16%, 19-23% and 16-18% of the label respectively. We conclude from the foregoing, together with identities [described in the preceding paper, Brearley and Hanke (1996) Biochem. J. 314, 215-225] of inositol phosphates found in this plant at a developmental stage associated with massive accumulation of InsP(6), that synthesis of InsP(6) from myo-inositol proceeds according to the sequence Ins3P --> Ins(3,4)P-2 --> Ins(3,3,6)P-3 --> Ins(3,4,5,6)P-4 --> Ins(1,3,4,5,6)P-5 --> InsP(6) in Spirodela polyrhiza. These results represent the first description of the synthetic sequence to InsP(6) in the plant kingdom and the only comprehensive description of endogenous inositol phosphates in any plant tissue. The sequence described differs from that reported in the slime mould Dictyostelium discoideum.