-Glycerolphosphate dehydrogenase activity and levels of glyceride-glycerol precursors in mouse mammary tissues

Differential oxidation of saturated and unsaturated fatty acids in vivo in the rat

The oxidation rates of lauric, myristic, palmitic, stearic, oleic, alpha-linolenic, linoleic, kappa-linolenic, dihomo-gamma-linolenic and arachidonic acids were studied by use of a radioisotope tracer technique in weanling rats at rest in a metabolism chamber over 24 h. Of the saturated fatty acids, lauric acid (12:0) was the most efficient energy substrate: the longer the chain length of the saturated fatty acids, the slower the rate of oxidation. Oleic acid (18:1) was oxidized at a remarkably fast rate, similar to that of lauric acid. Of the omega 6 essential fatty acids studied, linoleic acid (18:2 omega 6) was oxidized at a faster rate than any of its metabolites, with arachidonic acid (20:4 omega 6) being oxidized at the slowest rate. The rate of oxidation of gamma-linolenic acid (18:3 omega 3) was almost as fast as that of lauric and oleic acids.

Fatty acid specificities of microsomal acyltransferases esterifying positions-1 and -2 of acylglycerols in mammary glands from lactating rats

The acyl specificities of several acyltransferases located in the microsomal fraction of lactating rat mammary gland have been investigated using palmitate and oleate as substrates along with CoA, ATP and Mg(2+), bovine serum albumin and NaF. With either sn-glycerol 3-phosphate or dihydroxyacetone phosphate (plus NADPH) as acyl acceptor, phosphatidic acid containing palmitate preferentially esterified at position-2 and oleate at position-1 was the major product. Dihydroxyacetone phosphate and sn-glycerol 3-phosphate competitively inhibited each other's acylations, suggesting that a single enzyme might be responsible for both esterifications and oleate was the preferred substrate for the formation of acyldihydroxyacetone phosphate. The specificities of the acyl-CoA-1-monoacyl-sn-glycerol 3-phosphate and the acyl-CoA-2-monoacyl-sn-glycerol 3-phosphate acyltransferases were also studied. The specificities observed combined with the relative velocities of these reactions suggest that phosphatidic acid is formed in the mammary gland with the first acylation occurring at position-1 favouring oleate followed by the second acylation at position-2 favouring palmitate. This is consistent with the unusual structure found in the triacylglycerols of rat milk. When a mouse liver microsomal fraction was used the opposite specificities were observed consistent with the structure of the triacylglycerols of mouse liver. The microsomal acylation of the monoacyl-sn-glycerol 3-phosphocholines was also investigated. Although no marked acyl specificity could be detected when the 2-monoacyl-sn-glycerol 3-phosphocholine was used as the acyl acceptor, both oleate and linoleate were esterified in preference to palmitate to the 1-monoacyl-sn-glycerol 3-phosphocholine.

Triglyceride synthesis from dihydroxyacetone phosphate and palmitate by microsomes from mammary glands of lactating mice

Both di- and triglycerides were synthesized when microsomes isolated from mammary glands of lactating mice were incubated with dihydroxyacetone phosphate (DHAP), 1(-14)C)palmitate, ATP, CoASH, GSH, KF, MgC12, and NADPH. When NADH replaced NADPH, glyceride synthesis was very low. In the absence of either NADPH or NADH, DHAP was acylated to palmityl-DHAP. Since microsomes do not have glycerol 3-phosphate NAD:oxidoreductase activity , we inferred that glycerol 3-phosphate (GP) is not an intermediate in triglyceride biosynthesis from DHAP. This reductase, present in the cytosol, was active only with NADH. With the same concentration of either GP or DHAP, microsomes yielded essentially similar amounts of di- and triglycerides. Mitochondria, while capable of synthesizing palmityl-DHAP, did not produce di- and triglycerides.

Stimulatory effect of glucose upon triglyceride synthesis from acetate, decanoate, and palmitate by mammary gland slices from lactating mice

Slices prepared from the mammary glands of lactating mice incorporate only small amounts of (1--14C) acetate, (1--14C) decanoate, or (1--14C) palmitate into lipids. However, when glucose is added to the incubation medium, fatty acid incorporation is stimulated-13-fold from acetate, 17-fold from decanoate, and 2-fold from palmitate. Over 90% of the -14C activity in the lipid fraction is in triglycerides. Analysis of fatty acids in the triglycerides showed that almost all of the decanoate and the palmate were incorporated as intact molecules, while acetate yielded acids of varying chain lengths. The glucose stimulation of triglyceride synthesis is not solely due to its effect upon chain elongation but also could involve glyceride-glycerol availability, as well as other unknown factors. However, from measurements of the amounts of glycerol 3-phosphate in the tissue incubated in the presence of palmitate, it would appear that glyceride-glycerol availability is not rate limiting in triglyceride synthesis.

The relative utilization of the acyl dihydroxyacetone phosphate and glycerol phosphate pathways for synthesis of glycerolipids in various tumors and normal tissues

Rates of phosphatidate synthesis from dihydroxyacetone phosphate via acyl dihydroxyacetone phosphate or glycerol phosphate are compared in homogenates of 13 tissues, most of which are deficient in glycerol phosphate dehydrogenase (EC 1.1.1.8). In all tissues examined, dihydroxyacetone phosphate entered phosphatidate more rapidly via acyl dihydroxyacetone phosphate than via glycerol phosphate. Tissues with a relatively low rate of phosphatidate synthesis via glycerol phosphate, showed no compensating increase in the rate of synthesis via acyl dihydroxyacetone phosphate. The rates at which tissue homogenates synthesize phosphatidate from dihydroxyacetone phosphate via glycerol phosphate increase as glycerol phosphate dehydrongenase increase. Both glycerol phosphate dehydrogenase and glycerol phosphate: acyl CoA acyltransferase (EC 2.3.1.15) are more active than dihydroxyacetone phosphate : acyl CoA acyltransferase (EC 2.3.1.42). Thus, all the tissue homogenates possessed an apparently greater capability to synthesize phosphatidate via glycerol phosphate than via acyl dihydroxyacetone phosphate, but did not express this potential. This result is discussed in relation to in vivo substrate limitations.

Ultrastructure, steroidogenic potential, and energy metabolism of the Snell adrenocortical carcinoma 494. A comparison with normal adrenocortical tissue

Electron microscope studies were carried out with the adrenocortical carcinoma 494 and normal adrenal cortex tissue. The mitochondria of the tumor cells showed marked differences when compared with mitochondria from fasciculata cells of the normal adrenal cortex. These differences were primarily related to mitochondrial number and crista structure. Corticosterone production in isolated tumor cells was extremely low and neither ACTH nor dibutyryl cyclic AMP had any stimulatory effect. Normal adrenal cells showed at least a tenfold increase under identical conditions. In the presence of corticosteroid precursors the amount of corticosterone produced by the tumor cells was much less than that produced by normal cells. The results indicate a reduced capacity for 11beta-hydroxylation in the tumor mitochondria and a possible reduced capacity for biosynthetic steps before the 11beta-hydroxylation reaction. Glycolysis in isolated tumor cells was also lower than in normal cells. Isolated tumor mitochondria oxidized succinate normally with a good degree of coupling with phosphorylation. However, unlike normal adrenal mitochondria, the tumor mitochondria showed little or no oxygen uptake with other Krebs cycle substrates. These data suggest that the tumor mitochondria may be lacking in the flavoprotein dehydrogenases responsible for the oxidation of NADH and NADPH, although other components of the respiratory chain may be intact.