Oestrogen-induced cholesterol and fatty acid biosynthesis in Xenopus laevis liver during vitellogenic response

1. Oestradiol-17β induces livers of Xenopus laevis (South African clawed toad) to synthesize and secrete into the serum large quantities of the egg-yolk-protein precursor, vitellogenin. The peak of this response occurs 9–16 days after hormone treatment [Dolphin, Ansari, Lazier, Munday & Akhtar (1971) Biochem. J. 124, 751–758]. It is now shown that 6 days after hormone treatment a 120–160-fold stimulation of the synthesis of cholesterol and fatty acid compared with control values occurred. 2. A cell-free system, derived from Xenopus liver, which synthesizes squalene and fatty acid is described. By using this system, several hundredfold stimulation of incorporation of [¹⁴C]acetate into squalene was recorded 6 days after the administration of oestradiol-17β, compared with a 3–4-fold stimulation of incorporation of [³H]mevalonate compared with control values. It is argued that oestradiol-17β must affect enzyme(s) catalysing step(s) between acetate and mevalonate in the biosynthetic pathway to cholesterol. 3. In incubation of liver slices in vitro, most of the lipid and cholesterol synthesized in response to the steroid hormone was associated with those subcellular fractions that contained membranes. Moreover, pulse-labelling experiments in vivo showed that 70% of this lipid and cholesterol was retained in the liver. The remainder appeared in the serum, where it was equally distributed between vitellogenin and vitellogenin-free serum. 4. G.l.c. analyses of the cholesterol content of liver microsomal fractions of Xenopus laevis indicated that the cholesterol content was at least 50% higher in microsomal fractions obtained from livers that had been exposed to oestradiol-17β. Meanwhile, g.l.c. analysis of the lipid moiety of secreted vitellogenin showed that up to 35% of its lipid was cholesterol.     

Purification of rat liver S-adenosyl-l-methionine decarboxylase (Short Communication)

The amount of S-adenosyl-l-methionine decarboxylase present in rat liver was enhanced 17-fold by administration of methylglyoxal bis(guanylhydrazone),* a specific inhibitor of the enzyme. The enzyme was purified 1400-fold in 50% yield from such liver extracts by chromatography on columns of the inhibitor bound to Sepharose. The purified enzyme had no spermidine synthetase activity.     

Participation of propionate in cholesterol biosynthesis by rat liver

Incorporation of (2¹⁴C) propionate into cholesterol was demonstrated using rat liver slices and homogenates. The incorporation of (2¹⁴C) propionate was greater than that of (2¹⁴C) acetate. Using the same liver homogenate preparation (2 ¹⁴C) succinate and (2¹⁴C) pyruvate were incorporated into cholesterol to a lesser extent than (2¹⁴C) acetate and (2 ¹⁴C) propionate. Addition of unlabeled acetate failed to dilute the incorporation of (2 ¹⁴C) propionate. Incorporation of the 2 and 3 carbon atoms pf propionate were equal; little incorporation of the 1 carbon atom was demonstrable. These results indicate that propionate is an excellent source of 2 carbon units for isoprenoid biosynthesis; the intermediary pathway does not involve a common acetate pool nor can these results be satisfactorily explained by citric acid intermediary metabolism.     

The synthesis of medium-chain fatty acids by lactating-rabbit mammary gland studied in vitro (Short Communication)

The proportion of C(8:0) and C(10:0) fatty acids synthesized by the microsomal plus particle-free supernatant fraction from lactating rabbit mammary gland is enhanced at high protein concentrations. This fraction appears to contain a soluble high-molecular-weight factor that modifies the specificity of the fatty acid synthetase complex for termination of the growing acyl chain.     

The effects of lactate and acetate on fatty acid and cholesterol biosynthesis by isolated rat hepatocytes

1. The present study demonstrates that lactate and acetate stimulate fatty acid synthesis and inhibit cholesterogenesis by isolated rat hepatocytes. 2. Exposure of the intact cells to lactate increases the activity of acetyl-CoA carboxylase, as can be measured in homogenates of these cells. A similar effect by acetate was not observed. 3. Both acetate and lactate drastically increase the cellular level of citrate. 4. Possible mechanisms underlying the difference in response of fatty acid and cholesterol synthesis to an increase in substrate availability are discussed. Futhermore, a mechanism is proposed for the lactate effect on acetyl-CoA carboxylase.     

The apparent stimulation of proteolysis by adenosine triphosphate in tissue homogenates (Short Communication)

Reports that ATP promotes proteolysis in tissue homogenates were reinvestigated. Although ATP increased production of material reacting with ninhydrin or Folin-phenol reagent, ATP did not stimulate protein degradation in such extracts. AMP and adenosine acted similarly to ATP. Deamination of the added nucleotides to produce ammonia caused the increase in ninhydrin-positive material.     

Peroxisome proliferators enhance linoleic acid metabolism in rat liver. Increased biosynthesis of omega 6 polyunsaturated fatty acids

The alterations by peroxisome proliferators of metabolism of linoleic acid in rat liver were studied. Administration of P-chlorophenoxyisobutyric acid (clofibric acid) enhanced in vivo conversion of linoleic acid to its desaturated and/or elongated metabolites, 6,9,12-octadecatrienoic acid, 8,11,14-eicosatrienoic acid, and arachidonic acid, whereas the formation of 11,14-eicosadienoic acid was decreased. These changes observed in vivo were confirmed in vitro to be due to the increases in activities of delta 6 desaturation of linoleic acid to 6,9,12-octadecatrienoic acid (18.4 times), delta 8 desaturation of 11,14-eicosadienoic acid to 8,11,14-eicosatrienoic acid (3.4 times), and delta 5 desaturation of 8,11,14-eicosatrienoic acid to arachidonic acid (4.1 times). No considerable changes in activities of chain elongation of either linoleic acid or 6,9,12-octadecatrienoic acid were observed. The increases in the activities of three desaturations by clofibric acid were prevented by the treatment of rats with cycloheximide. The inductions of delta 6 and delta 5 desaturations were brought about by the treatment of rats with 2,2'-(decamethylenedithio)diethanol or di-(2-ethylhexyl)-phthalate, peroxisome proliferators structurally unrelated to clofibric acid, as well. These changes in metabolism of linoleic acid by clofibric acid were consistent with the changes in mass proportion of omega 6 fatty acids in hepatic lipid. Physiological significance of the marked changes in linoleic acid metabolism by peroxisome proliferators was discussed.