Different selectivities in acylation and methylation pathways of phosphatidylcholine formation in guinea pig and rat livers.

Radioactivity from intraperitoneally or intraportally injected 1-acyl-snglycero-3-phosphorylcholine, doubly labelled in either palmitoyl, glycerol or phosphoryl moities, was incorporated largely into disaturated or mixed disaturated-oligoenoic fractions of phosphatidylcholine in guinea pig liver. In rat liver the tetraenoic class was the most highly labelled and only very low radioactivity was recovered from disaturated or monoenoic species. The methylation of phosphatidylethanolamine to phosphatidylcholine, as judged by the incorporation of intraperitoneally injected L-[Me-14 C] methionine, involved selectively tetraenoic and polyenoic (greater than 4 double bonds) classes in rat liver. In guinea pig liver, methylation activity was much lower and led to the formation principally of dienoic classes of phosphatidylcholine. These experiments confirm the work of others that in rat liver, which has a high level of polyunsaturated classes of phosphatidylcholine, the "indirect" pathways of synthesis give rise chiefly to these classes. However, the priorties in guinea pig liver are different, since the levels of polyunsaturated classes are much lower, and although the "indirect" pathways of synthesis are operative they are directed mainly toward the formation of more saturated classes.