Oleic acid desaturation in Tetrahymena pyriformis

Fetal bovine serum concentration affects delta9 desaturase activity of Trypanosoma cruzi

Fetal bovine serum (FBS) is an important factor in the culture of Trypanosoma cruzi, since this parasite obtains and metabolizes fatty acids (FAs) from the culture medium, and changes in FBS concentration reduce the degree of unsaturation of FAs in phosphoinositides. When T. cruzi epimastigotes were cultured with 5% instead of 10% FBS, and stearic acid was used as the substrate, (9) desaturase activity decreased by 50%. Apparent K (m) and V (m) values for stearic acid, determined from Lineaweaver-Burk plots, were 2 microM and 219 pmol/min/mg of protein, respectively. In studies of the requirement for reduced pyridine nucleotide, (9) desaturase activity reached a maximum with 8 microM NADH and then remained constant; the apparent K (m) and V (m) were 4.3 microM and 46.8 pmol/min/mg of protein, respectively. The effect of FBS was observed only for (9) desaturase activity; (12) desaturase activity was not affected. The results suggest that decreased FBS in culture medium is a signal that modulates (9) desaturase activity in T. cruzi epimastigotes.

Temperature-induced membrane-lipid adaptation in Acanthamoeba castellanii

A method has been developed for the separation of the major membrane fractions of Acanthamoeba castellanii after growth at different temperatures. The acyl-lipid compositions of individual membrane fractions, microsomal membranes, plasma membrane and mitochondria were analysed after a shift in culture temperature from 30 degrees C to 15 degrees C. The major change in lipid composition observed was an alteration in the relative proportions of oleate and linoleate. This reciprocal change was seen in all the membrane fractions, but occurred most rapidly in the phosphatidylcholine of the microsomal fraction. Thus, there appears to be a rapid induction of delta 12-desaturase activity in A. castellanii after a downward shift in growth temperature. Changes were also seen in the proportions of the n-6 C20 fatty acids, with a decrease in the proportions of icosadienoate and increases of icosatrienoate and arachidonate. However, unlike the alteration in oleate/linoleate ratios, this change was not seen in all the individual lipids of each membrane fraction.

The effect of dietary sterol on the activity of fatty acid desaturases isolated from Tetrahymena setosa

Tetrahymena setosa has a nutritional requirement for micro amounts of sterol, a requirement which is also satisfied by relatively large amounts of either intact phospholipids or a mixture of unsaturated fatty acids normally found in these ciliates. Three microsomal fatty acyl-CoA desaturases have been isolated from T. setosa and partially characterized. These enzymes which can account for the formation of the majority of the ciliate's unsaturated fatty acids, include: a delta 9, a delta 12 and a delta 6 desaturase which catalyze the transformation of stearoyl-CoA to oleic acid, of oleoyl-CoA to linoleic acid and of linoleoyl-CoA to gamma-linolenic acid, respectively. The stearoyl CoA desaturase required NAD (or NADP), ATP and free CoA; the delta 6 and delta 12 desaturases required NADP, but not ATP or CoA. Cellular levels of the three desaturases were highest in mid-logarithmic phase cells and lowest in stationary phase cells. In order to determine if there was a relationship between the sterol requirement and the ability of the organism to desaturate, T. setosa was grown in a synthetic medium supplemented with either cholesterol or a phospholipid which permits growth in the absence of cholesterol, or with both phospholipid and cholesterol. Cells grown with phospholipid alone had only half as much stearoyl-CoA and oleoyl-CoA desaturase activity as cells of identical culture age grown either on cholesterol alone or on cholesterol plus phospholipid.

Functional association of a monoacylglycerophosphocholine acyltransferase and the oleoylglycerophosphocholine desaturase in microsomes from developing leaves

The biosynthesis of linoleic acid has been investigated, using oleoyl-CoA as a substrate, in microsomal preparations from young leaves of Pisum sativum. Oleoyl moieties from oleoyl-CoA were preferentially acylated to lysophosphatidylcholine by an acyltransferase to produce an oleoylglycerophosphocholine. Kinetic data are presented which argue for a direct desaturation of the oleoyl moieties of this oleoyl glycerophosphocholine to linoleoyl moieties. There was no evidence of a subsequent acyltransfer of linoleoyl moieties either to form thioesters or oxygen esters in other complex lipids. The kinetics were also consistent with a functional coupling of the lysophosphatidylcholine acyltransferase with the oleate desaturase. There was little exchange of the oleoyl glycerophosphocholine from the bulk membrane lipid with that newly synthesised by the lysophosphatidylcholine acyltransferase. Rather, the newly synthesised oleoylglycerophosphocholine seemed to be directly channelled to the vicinity of the desaturase. The results are discussed in the context of 'metabolite channelling'. The consequences for desaturase activity and its regulation are also examined.

A cytochrome b5 electron transport chain in Tetrahymena

Tetrahymena microsomes contain cytochrome b5 and an NADH-dependent cytochrome b5 reductase, but these proteins are present at only one-tenth the levels observed in rat liver microsomes. We show that both proteins can be partially purified by techniques developed for the rat liver proteins. We can show that cyanide inhibits the rate of exhaustion of NADH, and therefore reoxidation of cytochrome b5 by microsomes, and that stearoyl CoA enhances the rate of reoxidation of the cytochrome. Also, we find that a fragment of rat liver NADH-cytochrome b5 reductase can restore NADH-dependent cytochrome b5 reduction to Tetrahymena microsomes which have been treated with N-ethylmaleimide to eliminate endogenous reductase activity. These results indicate that there is considerable resemblance between the rat and Tetrahymena systems, and that desaturation of stearoyl and oleoyl groups may occur in Tetrahymena via pathways similar to those known in liver.