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Envelope membranes were isolated from potato tuber amyloplast by a discontinuous sucrose density gradient and high speed centrifugation. These membranes catalyzed the transfer of [14C]glucose from UDP-[14C]glucose to endogenous sterol acceptors and, in turn, catalyzed the esterification of steryl glucosides with fatty acids from an endogenous acyl donor. The synthesis of steryl glucosides was stimulated in the presence of Triton X-100, while formation of acyl steryl glucosides was inhibited by the detergent. However, in the presence of an added sterol acceptor and Triton X-100, the inhibition of acyl steryl glucoside synthesis was overcome by the addition of phosphatidylethanolamine. The enzyme involved in steryl glucoside formation was solubilized by treatment of the envelope membranes with 0-3% Triton X-100. The solubilized enzyme had an almost absolute requirement for sterol acceptors.

Steryl glucoside (SG) and acylated steryl glucoside (ASG) synthesis was investigated in amyloplast membranes from young, intermediate and mature potato tubers. The synthesis ratio SG/ASG was lowest in young tubers (3:2) and highest in mature tubers (6:1). About a 3-fold stimulation of [14C] glucose incorporation into a mixture of SG was observed in amyloplast membranes from mature tubers in the presence of β-sitosterol, while radioactivity incorporation in young tubers was unaffected, thus indicating that different availabilities of endogenous acceptors occur in the membranes. The enzymes involved in sterol modification exhibit a different behavior towards Triton X-100, depending on the developmental stage of the tubers. Low concentrations of the detergent (0.045%) are required to stimulate the enzymes present in young tuber membranes (2-fold). On the other,hand, 0.15% of Triton increased the enzymatic activity in mature tubers 5-fold. These results, together with those obtained after studies of pH dependence, could be related to the lipidic structure of the vesicles formed at different developmental stages of the tubers. It is concluded that the major changes in the enzymatic activities occur as a consequence of the sterol acceptors and acyl donor content during potato tuber growth.