Fatty acid synthesis in rat testes injected intratesticularly or incubated with 1- 14 C Acetate 1

Biosynthesis of long-chain polyenoic acids from arachidonic acid in cultures of enriched spermatocytes and spermatids from mouse testis

Primary spermatocytes (PS), round spermatids (RS) and condensing spermatids (CS) from mouse testes were enriched on Sta-Put 1 X g density gradients and cultured for 22 or 44 hr in the presence of [1-14C]arachidonate. Mass and radioactivity were measured by gas radiochromatography of constituent fatty acids of the various complex lipid classes fractionated by thin layer chromatography. Patterns and levels of incorporation were compared with those of whole testis, both in vitro and in vivo. The 20:4, 22:4, 22:5, 24:4 and 24:5 of the germinal cells contained levels of radioactivity in each lipid class which were consistent with an important role for the germinal cells in long-chain polyenoic acid (LCPA) metabolism. Cells which represented later stages of spermatogenesis (RS, CS) incorporated much higher percentages and absolute amounts of radioactivity into the fatty acids derived from 20:4 by elongation-desaturation pathways than did PS or whole testis in vitro. These differences were most pronounced in triacylglycerol of CS. Distributions of mass and radioactivity among lipid classes suggest synthesis of triacylglycerol by CS with a high degree of specificity for 22 or 24 carbon LCPA at the sn-3 position.

Composition of, and [14C]acetate incorporation into, lipids of rat Sertoli cells in culture

Primary cultures of rat Sertoli cells were used for the determination of their lipid and fatty acid composition and for a demonstration of their ability to incorporate 14C from [14C]acetate into fatty acids esterified in the cellular lipids. Similar fatty acid compositions were observed in 9-day cultures of Sertoli cells of rats aged 3, 4 or 5 weeks. Cultures of Sertoli cells of 5-week-old rats had relatively less palmitic, linoleic and arachidonic acids and more palmitoleic, stearic, oleic and docosatetraenoic acids than non-cultured cells from the same pool. No difference between these was observed in phospholipid concentration, but the cultured cells had a greater concentration of both total cholesterol and triacylglycerols than did the non-cultured cells. 14C from [14C]acetate added to the culture medium on the 7th (48-h incubation) or 8th (24-h incubation) day was incorporated into fatty acids esterified in cellular lipids. Major incorporation was into phospholipids and triacylglycerols, but cholesterol also was labeled. The fatty acids containing most of the 14C were the saturated fatty acids, oleic acid and docosatetraenoic acid (products of both de novo synthesis and elongation reactions).

Fatty acid synthesis in isolated spermatocytes and spermatids of mouse testis

In vitro incorporation of [1-14C] acetate into fatty acids and lipid classes of spermatocytes, round spermatids and condensing spermatids enriched by Staput 1 X g sedimentation was measured by thin layer and gas radiochromatography. All three cell fractions showed the full range of de novo synthetase, elongation and desaturase activities necessary for biosynthesis of fatty acids characteristic of mouse testis, but synthesis of fatty acids of greater than 16 carbons declined with progressive stages of differentiation. The magnitudes and patterns of distribution of fatty acid synthesis in the germinal cells were similar to those of whole testis incubated in vitro or injected in vivo with [14C]acetate. On the other hand, complex lipid synthesis was much more variable and incorporation into triacyglycerol was generally much lower in dispersed germinal cells than in whole testis in vitro or in vivo. Cells remained viable throughout the 15-hr incubation. Thus, isolated germinal cells are fully capable of synthesizing their constituent fatty acids, including the long-chain polyenoic acids which they accumulate, but the intratubular environment or association with Sertoli cells may be necessary for maintenance of adequate complex lipid synthesis.

Metabolism of eicosa-11,14-dienoic acid in rat testes. Evidence for delta8-desaturase activity

The metabolism of [14C]eicosa-11,14-dienoic acid was investigated in rat testes in vivo and in vitro. Intratesticular injection of [1-14C]eicosa-11,14-dienoic acid resulted in the appearance of radioactivity (4-30% of 14C in total fatty acids) in 20-carbon trienoic fatty acids and a small amount (2-3.5%) in arachidonic acid. Analysis of the 20-carbon trienoic acid fraction by ozonolysis indicated that 15 to 34% of the 14C in this fraction was in an 8-carbon fragment originating from eicosa-8,11,14-trienoic acid. The rest (66 to 84%) was in a 5-carbon fragment, presumably originating from eicosa-5,11,14-trienoic acid. Incubation of testicular tissue minces or microsomes with [1-14C]eicosa-11,14-dienoic acid yielded labeled eicosa-8,11,14- and eicosa-5,11,14-trienoic acids in proportions similar to those obtained in vivo. Added unlabeled acetate had no effect on the formation of [14C]eicose-8,11,14-trienoic acid in vitro. Therefore, it is unlikely that the labeled eicosa-8,11,14-trienoic acid arose from elongation of octadeca-6,9,12-trienoic acid with labeled acetate derived from bio-oxidation of the labeled substrate. These results are compatible with a limited desaturation of eicosa-11,14-dienoic acid to eicosa-8,11,14-trienoic acid and provide evidence for delta8 desaturate activity in rat testis.

Fatty acid synthesis from 2-14C-acetate in rat testis mitochondrial and cytosol fractions in vitro

An in vitro system for acetate incorporation into fatty acids by the mitochondrial and the cytosol fractions of rat testis is described. The rate of incorporation of acetate into fatty acids was twice as fast with the mitochondrial as with the cytosol fraction; both systems were stimulated in the presence of adenosine triphosphate, reduced nicotinamide adenine dinucleotide phosphate, coenzyme A, and MgC1(2). The optimum pH was between 7.0-7.5 for the mitochondrial fraction and between 6.5-8.0 for the cytosol fraction. Radio gas chromatography showed that palmitic acid was the most highly labeled acid, followed by stearic acid, in the mitochondrial fraction in accord with the pathway of de novo fatty acid synthesis. Some of the labeled acetate was also incorporated into the 16:1 and 18:1 fatty acids of this fraction. Distribution of radioactivity among the mitochondrial lipid classes was highest in the phospholipids and monoglycerides, followed by diglycerides and cholesterol; little radioactivity was present in the triglyceride fraction. These observations are in accord with studies of the incorporation of labeled metabolites into testicular lipids following intratesticular injection and indicate the validity of the in vitro system for studies of specific reactions occurring in vivo.

Studies in vitro of lipogenesis in rat testicular tissue

Testicular tissue was shown to contain the full complement of enzymes required for de novo synthesis of fatty acids. The enzymes capable of snythesizing palmitic acid from citrate, acetate, or acetyl CoA were found to be present in the soluble (cytoplasmic) fraction. These included fatty acid synthetase, acetyl CoA carboxylase, citrate-cleavage enzyme, malic enzyme, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase. Optimal conditions for assaying activities of fatty acid synthetase and acetyl CoA carboxylase in the soluble fraction from rat testes were established, and the activities of these two enzymes were determined to be 0.54 +/- 0.1 and 0.030 +/- 0.002 (nmoles of substrate incorporated into fatty acid per min per mg of soluble fraction protein), respectively. The activities of citrate-cleavage enzyme, malic enzyme, and the glucose-6-phosphate dehydrogenase/6-phosphogluconate dehydrogenase pair were also measured. The activities were 6.0 +/- 0.7, 34.9 +/- 4.2, and 29.9 +/- 9.3 nmoles/min/mg, respectively.