Nature of the Fatty Acid Synthetase Systems in Parenchymal and Epidermal Cells of Allium porrum L. Leaves

Abstract

Fatty acid synthesis was compared with cell-free extracts of epidermis and parenchyma of A. porrum L. leaves. Parenchyma extracts had the major fatty acid synthase (FAS) activity (70-90%) of the whole leaf; palmitic acid was also the major fatty acid synthesized when acetyl-CoA was the primer, but when acetyl-acyl carrier protein (ACP) was employed, C18:0 and C16:0 were synthesized in equal proportion. With the epidermal FAS system when either acetyl-CoA or acetyl-ACP was tested in the presence of labeled malonyl-CoA, palmitic acid was the only product synthesized. Specific activities of the FAS enzyme activities were determined in both tissue extracts. The properties of malonyl-CoA:ACP transacylase were examined from the 2 tissues. The MW estimated by Sephadex G-200 chromatography were 38,000 for the epidermal enzyme and 45,000 for parenchymal enzyme. The optimal pH was for both enzymes 7.8-8.0 and the maximal velocity 0.4-0.5 .mu.mol/mg protein per min. These enzymes had different affinities for malonyl-CoA and ACP. For the malonyl-CoA:ACP transacylase of epidermis, the Km values were 5.6 and 13.7 .mu.M for malonyl-CoA and ACP, respectively, and 4.2 and 21.7 .mu.M for the parenchymal enzyme. Apparently the FAS system in both tissues are nonassociated, the malonyl-CoA:ACP transacylases are isoenzymes, and both in epidermis and in parenchyma tissue 2 independent FAS systems occur. Evidence would suggest that .beta.-ketoacyl-ACP synthase II is present in the parenchymal cells but missing in the epidermal cell.