Comparison of acetate- and pyruvate-dependent fatty-acid synthesis by spinach chloroplasts

✍ Scribed by Jutta Springer; Klaus-Peter Heise

• Publisher: Springer-Verlag
• Year: 1989
• Tongue: English
• Weight: 542 KB
• Volume: 177
• Category: Article
• ISSN: 0032-0935
• DOI: 10.1007/bf00403601

✦ Synopsis

In recent studies using intact chloroplasts of spinach (Spinacia oleracea L.) to investigate the accumulation of acetyl-CoA produced by the activity of either acetyl-CoA synthetase (EC 6.2.1.1) or the pyruvate-dehydrogenase complex, this product was not detectable. These results in combination with new information on the physiological levels of acetate and pyruvate in spinach chloroplasts (H.-J. Treede et al. 1986, Z. Naturforsch. 41C, 733-740) prompted a reinvestigation of the incorporation of [1-14C] acetate and [2-14C] pyruvate into fatty acids at physiological concentrations.

The Km for the incorporation into fatty acids was about 0.1 mM for both metabolites and thus agreed with the values obtained by H.-J. Treede et al. (1986) for acetyl-CoA synthetase and the pyruvate dehydrogenase complex. However, acetate was incorporated with a threefold higher Vm~ x. Saturation for pyruvate incorporation into the fattyacid fraction was achieved only at physiological pyruvate concentrations (<1.0 mM). The diffusion kinetics observed at higher concentrations may be the result of contamination with derivates of the labeled substrate. Competition as well as double-labeling experiments with [3H]acetate and [2-14C]pyruvate support the notion that, at least in spinach, chloroplastic acetate is the preferred substrate for fatty-acid synthesis when both substrates are supplied concurrently (P.G. Roughan et al., 1979b, Biochem. J. 184, 565-569).

Experiments with spinach leaf discs confirmed the predominance of fatty-acid incorporation from acetate. Radioactivity from [1-14C]acetate appeared to accumulate in glycerolipids while that from [2-1~C]pyruvate was apparently shifted in favor of the products of prenyl metabolism.