Stearyl-ACP

All plant extracts that synthesize fatty acids from acetyl-CoA and mal-onyl-CoA are completely dependent upon the presence of ACP (Stumpf, 1977 Ohlrogge et al., 1979). In addition, its role as the thioester component of the primary elongation substrate, namely, palmityl-ACP, and the first desaturation substrate, stearyl-ACP, is well documented (Jaworski and Stumpf, 1974 Jaworski et al., 1974). Two plant ACPs have been isolated and characterized, one from avocado mesocarp and one from spinach leaves (Simon et al., 1967). The characteristics of these ACPs, as compared to prokaryotic ACPs, are given in Table I. 

As noted in Table II, all proplastids or chloroplasts synthesize oleic acid as their main unsaturated fatty acid from [ Cjacetate. Since Ohlrogge et al. (1979) have shown that all the ACP of a leaf cell is localized in the chloro-plast, it follows that the only site of stearyl-ACP desaturase must be the chloroplast. In addition, the highly specific acyl-ACP hydrolase must also be localized in the same organelle. The most effective substrate for acyl-ACP hydrolase is oleoyl-ACP, with stearoyl-ACP, palmitoyl-ACP, myristoyl-ACP, and lauroyl-ACP decreasing in activity in that order (Ohlrogge et al., 1978b). The consistent presence of the desaturase and the hydrolase in chloroplasts will be discussed in Section III. 

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