The Energy‐Dependent Incorporation of Serine into Phospholipids of Isolated Rat Liver Mitochondria

A Mg++‐requiring, energy‐dependent incorporation of labelled serine by isolated rat liver mitochondria has been studied. Possible contributions to the incorporation by bacteria and microsomes are negligible. The serine is incorporated mostly into phosphatidylserine (85%) with small amounts into phosphatidylethanolamine (7%). The energy requirement is satisfied by ATP which can be generated via oxidative phosphorylation or provided exogenously. Hypothetical high energy intermediates of oxidative phosphorylation are not obligatory. Subfractionation of the mitochondria following the incorporation of serine shows that the radioactivity is associated mostly with the outer membrane. The isolated inner and outer membranes also incorporate serine but not as efficiently as intact mitochondria. The data is discussed in relation to the biosynthesis of the mitochondrial membranes.