Fatty acid biosynthesis in human erythrocytes: evidence in mature erythrocytes for an incomplete long chain fatty acid synthesizing system.

The synthesis of long chain fatty acids from acetate in mammalian, extramitochondrial ("solu-ble") systems requires three enzymatic steps: 1) the activation of acetate to form acetyl-coenzyme A (acetyl-CoA), 2) the carboxylation of acetyl-CoA by the enzyme acetyl-CoA carboxylase to form malonyl-coenzyme A (malonyl-CoA), and 3) the subsequent complex series of serial condensations of malonyl-CoA to the elongating acyl-CoA compound [the intermediates may be bound to acyl carrier protein rather than CoA (1)] by the enzyme palmitate synthetase to form the resultant long chain fatty acid, shown in Table I. The synthesis of long chain fatty acids from acetyl-CoA has been studied extensively in enzyme systems from a number of sources, including rat, beef, and human adipose tissue (2-4), avian and mammalian liver (5-7), rat lactating mammary gland (8, 9), rat brain (10), yeast (11), plants (12), insects (13, 14), and bacteria (15). The formation of acetyl-CoA from acetate, although less extensively studied as an initial step in fatty acid biosynthesis, has been demonstrated in mam-malian liver (16), heart (17, 18), brain (19), and adipose tissue (20). In the human, fatty acid synthesis has been shown to occur by the malonyl-CoA pathway in adipose tissue (4) and possibly in liver (21). The human erythrocyte contains a significant quantity of long chain fatty acids, largely in the form of complex structural lipids (22-24). Both immature mammalian and nucleated avian eryth-rocytes have been shown to possess the ability to synthesize long chain fatty acids from acetate, but this ability is apparently lost in the mature mammalian red cell (25-28). The present study was undertaken to extend the information available on the mechanism of fatty acid synthesis in this readily accessible tissue and to define the metabolic lesion responsible for the reported inability of these cells to synthesize long chain fatty acids. Methods Preparation of substrate. The following substrates, cofactors, and enzymes were purchased commercially: ATP,1 TPN,1 TPNH,1 DPNH,l glucose-6-phosphate dehydrogenase,1 coenzyme A,2 acetic acid-l-14C-anhydride,s bicarbonate->C,3 and malonic acid-2-14C.2 Acetyl-CoA and acetyl-1-14C-CoA were synthesized by the method of Simon and Shemin (29) and malonyl-CoA and malonyl-2-14C-CoA by the method of Trams and Brady (30). Preparation of enzymes. Hemolysates were prepared from heparinized blood from normal, nonfasting, adult volunteers without regard to diet. All operations were carried out at 0 to 4° C. The erythrocytes were sepa