Factors effecting the solubilisation of stearoyl-coA desaturase of hen liver microsomes

The regulation of stearoyl-CoA desaturase (SCD)

The stearoyl-CoA desaturase gene family encodes stearoyl-CoA desaturase, the key enzyme involved in the biosynthesis of unsaturated fatty acids, as well as in the regulation of this process. Because of the important role that the SCD gene product plays in fat cell metabolism, future studies on SCD1 gene expression could provide new insights into the role of fatty acids in cellular regulation, metabolism, and gene expression both in normal and disease states. In addition, the SCD gene family can be used as a model to study mechanisms of cellular differentiation, tissue-specific gene expression, and dietary and hormonal regulation of gene expression.

Membrane lipid changes in erythrocytes, liver and kidney in acute and chronic experimental liver disease in rats

Lipid molecules in lipoprotein surfaces exchange with their counterparts in cell plasma membranes. In human or experimental liver disease, plasma lipoprotein surfaces are enriched in cholesterol and deficient in arachidonate; corresponding alterations occur in membrane lipids of erythrocytes. To determine whether similar changes take place in membranes of nucleated cells, the lipid content of plasma and of erythrocyte, liver and kidney membranes was measured in rats with acute (3-day) galactosamine-induced hepatitis or chronic (3-week) biliary obstruction. In both models of liver injury the cholesterol:phospholipid ratio in plasma and in erythrocytes was significantly increased (P less than 0.001). Although this ratio was also elevated in liver and kidney microsomes, only in liver microsomes of obstructed rats was the increase significant (P less than 0.001). However, the cholesterol:phospholipid ratio of kidney brush-border membranes, was significantly higher in bile-duct-ligated rats; presumably, compensating mechanisms limit cholesterol accumulation in intracellular membranes. Kidney brush-border membranes from obstructed rats were deficient in arachidonate as were plasma and erythrocytes. However, arachidonate levels were unchanged in kidney microsomes; renal delta 6-desaturase, the rate-limiting enzyme in the conversion of linoleic acid to arachidonic acid, was increased by 50% (P less than 0.001) and may have counteracted a reduced supply of exogenous lipoprotein arachidonate. We conclude that in experimental liver disease lipoprotein-induced lipid abnormalities can occur in renal membranes, although compensatory mechanisms may operate; the alterations seen, cholesterol accumulation and arachidonate depletion, would be expected to interfere with sodium transport and prostaglandin production, respectively. Our findings support the hypothesis that lipid abnormalities in kidney membranes contribute to the renal dysfunction which is a frequent complication of human liver disease.

Some properties, including the substrate in vivo, of the delta 9-desaturase in Micrococcus cryophilus

The delta 9-desaturase of the psychrophilic bacterium Micrococcus cryophilus is shown to be a membrane-bound enzyme that is probably linked to a cyanide- (and azide-) sensitive respiratory chain with oxygen as the final acceptor. It has a pH optimum of 8.7 and contains an essential thiol group, but has no special ion requirements. The desaturase activity of washed membranes could not be increased by adding supernatant or NADH and NADPH, possibly owing to the endogenous generation of reduced cofactors by the membranes. The substrate for the desaturase is not acyl-CoA and is probably not acyl-acyl-carrier protein. Evidence is presented that the substrate in vivo is saturated phospholipid and a scheme for the possible routes of incorporation of exogenous stearic acid into oleoyl-phospholipid is presented.

Biosynthesis of sphingomyelin from erythro-ceramides and phosphatidylcholine by a microsomal cholinephosphotransferase

Mouse liver microsomes were shown to be active in the synthesis of sphingomyelin from ceramide and phosphatidylcholine in a reaction independent of CDPcholine. The conversion was not inhibited by calcium chelating reagents, and no evidence for the involvement of phospholipase C activity in the transformation could be adduced. Activity was also demonstrated in monkey liver and heart microsomes. Mouse brain microsomes produced a sphingomyelin analogue, tentatively identified as ceramide phosphorylethanolamine, but not sphingomyelin. Both [14C]ceramide and [G-14]phosphatidylethanolamine were precursors of the brain product, while phosphatidylcholine was inactive. Progress in the partial characterization of the liver enzyme is also described.

Cyanide sensitivity and induction of the microsomal oleoyl-CoA desaturase of potato tuber

1. The properties of oleoyl-CoA desaturase, induced in microsomal fractions by the 'ageing' treatment of potato tuber slices (aeration of slices for 3-18 h), were investigated to study the effect of cyanide on desaturation and cycloheximide on the induction of the desaturase. 2. The electrons needed for the desaturation can be supplied either by NADH or NADPH, but ascorbate can also drive the reaction; experiments with CO suggested that cytochrome P-450 was not involved in the desaturation. 3. A strong inhibition of the desaturation by potassium cyanide was observed with each of the electron donors; total inhibition was noticed with 1 mM KCN; low concentrations (0.1 mM) caused 50% inhibition of the desaturation. 5. The variation of the oleoyl-CoA desaturase activity during the ageing process and the drop in this activity in aged slices treated by cycloheximide indicate that the enzyme undergoes an active turnover.