The Mechanism of Formation of Polyunsaturated Fatty Acids by Photosynthetic Tissue

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Abstract
Labelled oleic acid, which is used as a substrate in studying the conversion of oleate into linoleate by Chlorella chloroplasts, is rapidly incorporated into the phospholipids. The phosphatidyl choline fraction accounts for almost all of the phospholipid label. Desaturation to linoleate lags behind the incorporation into lipids, but the newly synthesized linoleate remains esterified to phosphatidyl choline.[1‐14C]Oleoyl‐phosphatidyl choline, incubated with chloroplasts, is converted into [1‐14C]‐linoleoyl‐phosphatidyl choline, but considerable difficulties arise in presenting the lipid to the enzyme in an acceptable form. Oleoyl‐phosphatidyl choline formed in situ, however, does not present these difficulties and is desaturated at a faster rate than the CoA derivative under conditions in which unesterified fatty acid cannot be “activated”, and hence cannot be desaturated. We conclude that the precursor fatty acid can be transferred directly from the lipid to the desaturase enzyme and the product immediately accepted by a lipid to form a “desaturation‐acylation” cycle. Our data neither prove, nor exclude the possibility that the fatty acid is transformed while still attached to the lipid in ester linkage.

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