INCORPORATION OF SERINE-14C AND ETHANOLAMINE-14C INTO NITROGEN-CONTAINING PHOSPHATIDES AND EFFECTS OF MEDIUM CONTAINING ETHANOLAMINE ON PHOSPHATIDE BIOSYNTHESIS IN EXCISED TOMATO ROOTS

Abstract

Work was designed to study the decarboxylation of serine and biosynthesis of ethanolamine as part of an investigation into the replacement of vitamin B6 by ethanolamine in the nutrition of excised tomato roots in sterile culture. A working hypothesis to explain the replacement involves repression, by ethanolamine, of a postulated vitamin B5 dependent serine decarboxylase yielding ethanolamine. Evidence is presented, from studies using uniformly labelled serine-14C serine-1-14C, serine-3-14C, and ethanolamine-l,2-14C for the incorporation of serine and ethanolamine into the corresponding phosphatides, for the occurrence of a pathway whereby phosphatidyl serine is decarboxyl-ated to phosphatidyl ethanolamine, which is further methylated to phosphatidyl choline, and for the existence of more than one molecular species of phosphatidyl ethanolamine. Experiments are reported in which uniformly labelled serine-14C was supplied in the presence and absence of the inhibitors ethionine, p-chloromercuribenzoate, or diethylethanolamine. The results suggest that in the excised tomato root a pathway of choline biosynthesis other than the phosphatide pathway may exist. Some evidence is given that the replacement of vitamin B6 by ethanolamine decreases the decarboxylation of phosphatidyl serine.