Lipid Metabolism In Mammals Research Articles

The bioinorganic chemistry of chromium(III)

The biochemistry of chromium(III) has been a poorly understood field of endeavor. Despite four decades of investigation, only recently has a somewhat clear picture of the role of Cr been refined. Chromium(III) is required for proper carbohydrate and lipid metabolism in mammals, although chromium deficiency is difficult to achieve. Conditions that increase circulating glucose and insulin concentrations increase urinary chromium output. Chromium is excreted after an insulin challenge in the form of the oligopeptide chromodulin. Chromodulin may be the key to understanding the role of chromium at a molecular level as the molecule has been found to bind to activated insulin receptor, stimulating its kinase activity. An examination of the history of studies of chromium picolinate and glucose tolerance factor illustrates the difficulties and problems associated with biochemical studies dealing with chromium(III).

Hepatic fatty acid metabolism in pigs and rats: major differences in endproducts, O2 uptake, and beta-oxidation.

Models of mammalian hepatic lipid metabolism are based largely on observations made in adult rats, emphasizing ketogenesis as a primary adjunct to mitochondrial beta-oxidation. Studies using piglets have illustrated the divergent nature of intermediary metabolism in this model, wherein ketogenesis and beta-oxidation are small and acetogenesis is an important route of fuel carbon flux. To clarify potential species differences in hepatic lipid metabolism and its control, we compared O2 consumption and metabolic end products in fasted pig and rat liver homogenates treated with 1-[14C]C16:0. Carboxyl carbon accumulation in acid-soluble products (ASP) plus CO2 was threefold greater and O2 consumption was twofold greater in rats (P < 0.05). Unlike rats, pigs showed negligible carboxyl carbon accumulation in ketone bodies (3-7% of ASP), whereas acetate was a carboxyl carbon reservoir in both animals (17-31% of ASP in pigs). Malonate increased (approximately 2-fold) and antimycin/rotenone decreased (40-60%) radiolabel accumulation in acetate. These data concur with the hypotheses that comparatively low hepatic beta-oxidative flux in piglets is partially related to a smaller metabolic rate and that substantial acetogenesis occurs intramitochondrially in both pigs and rats.

A biologically active form of chromium may activate a membrane phosphotyrosine phosphatase (PTP).

Chromium is essential for proper carbohydrate and lipid metabolism in mammals, although the mechanism of this action has previously proved elusive. Low-molecular-weight chromium-binding protein (LMWCr), a biologically active form of chromium in mammals, potentiates the effect of insulin on the conversion of glucose into lipid and into carbon dioxide in isolated adipocytes. Kinetics studies indicate that LMWCr isolated from bovine liver activates phosphotyrosine phosphatase (PTP) activity in adipocyte membranes while having no intrinsic phosphatase activity. This activation is directly proportional to the amount of added LMWCr. The pattern of inhibition of this activity in the presence of a number of known phosphatase inhibitors suggests the involvement of a membrane phosphotyrosine phosphatase similar to PTP1A' or PTP1B. We propose that chromium plays a biological role in the activation of a membrane phosphotyrosine phosphatase.

Characterization and chromosomal localization of the human homologue of a rat AMP-activated protein kinase-encoding gene: a major regulator of lipid metabolism in mammals

AMP-activated protein kinase (AMPK) phosphorylates and inactivates acetyl-CoA carboxylase and β-hydroxy β-methylglutaryl-coenzyme A (HMG-CoA) reductase which are the major enzymes involved in fatty acid and lipid biosyntheses. The AMPK gene from rat ( rAMPK) has recently been cloned [Carling et al. J. Biol. Chem. 269 (1994) 11442–11448]. In order to study the structure and function of the human AMPK gene (h AMPK), we have cloned the gene, and report in this communication its nucleotide (nt) sequence, tissue distribution and chromosomal location. Our results show that the ORF of h AMPK encodes 552 amino acids (aa) (62.250 kDa) and is highly conserved with r ampk with identities of 97.3 and 90% at the aa and nt levels, respectively. The h AMPK gene bears homology to a yeast protein kinase-encoding gene ( snf1) that regulates carbohydrate metabolism, and also with three other genes encoding SNFl-like kinases from different plant species, namely Arabidopsis thaliana, Hordeum vulgare and Secale cereale. As determined by fluorescent in situ hybridization of a human metaphase chromosome spread, h AMPK maps to chromosome lp31. The size of the h AMPK transcript is 8.5 kb and the transcription start point ( tsp) is located approx. 46 bp upstream from the ATG codon. While 10–15% of AMPK is alternatively spliced in most tissues of the rat, our RT-PCR analyses of the h AMPK mRNA did not reveal the presence of any alternatively spliced form of the gene in human tissues. An interesting aspect of AMPK is that its expression, unlike in rat liver, could not be detected in human liver, and thus the purported role of the gene in controlling fatty-acid synthesis in the human liver remains to be determined.

Mammalian AMP-activated protein kinase shares structural and functional homology with the catalytic domain of yeast Snf1 protein kinase.

The AMP-activated protein kinase is responsible for the regulation of fatty acid synthesis by phosphorylation of acetyl-CoA carboxylase. It may also regulate cholesterol synthesis via phosphorylation and inactivation of hormone-sensitive lipase and hydroxymethylglutaryl-CoA reductase. We have purified the AMP-activated protein kinase 14,000-fold from porcine liver. The 63-kDa catalytic subunit co-purifies with two proteins of 40 and 38 kDa that may function as subunits. Partial amino acid sequence of the 63-kDa subunit revealed a striking homology with the catalytic domain of the yeast protein kinase transcriptional regulator Snf1 and its plant homologs. The Snf1 (72 kDa) and Snf4 (36 kDa) complex was also purified and found to phosphorylate the AMP-activated protein kinase peptide substrate, HMRSAMSGLHLVKRR-amide, but was not activated by AMP. Both Snf1/4 and the AMP-activated protein kinase phosphorylate and inactivate yeast acetyl-CoA carboxylase in vitro. These results indicate that during evolution the catalytic domain sequences of the Snf1 protein kinase subfamily have been exploited in the control of mammalian lipid metabolism and raise the possibilities that the AMP-activated protein kinase may have other substrates involved in regulating gene expression pathways, as well as Snf1 homologs participating in the control of lipid metabolism in many eukaryotic organisms.

New Approaches in Peroxisomal Disorders

The peroxisome is a subcellular organelle with important functions in plants and protozoa, which during the last decade has also been shown to have a role in mammalian lipid and amino acid metabolism. These functions include steps in the synthesis of ether lipids and bile acids and fatty acid beta-oxidation, particularly those of very long chain fatty acids. The proposition that the peroxisome carries out significant functions in man is highlighted by the fact that lack of this organelle is associated with severe abnormalities in many human organs. Human peroxisomal disorders are now grouped into three general categories. In the first group, peroxisomes are lacking or reduced in number. This group includes the Zellweger cerebro-hepato-renal syndrome, neonatal adrenoleukodystrophy, infantile Refsum's disease and hyperpipecolic acidemia. These patients lack the capacity to synthesize ether lipids and to oxidize very long chain fatty acids or phytanic acid, and they show abnormally high levels of pipecolic acid and bile acid intermediates. These patients rarely survive early childhood, have severe neurological deficits and multiple malformations. A second group includes the 'pseudo-Zellweger' syndrome and the rhizomelic form of chondrodysplasia punctata. Here the peroxisomal structure is intact, but there is deficient function of several peroxisomal enzymes. The third group includes X-linked adrenoleukodystrophy, acatalasemia and 'adult' Refsum's disease. The peroxisomal structure is intact, and the defect in each instance is thought to involve a mutation which affects a single peroxisomal enzyme. Peroxisomal disorders are of current interest because they occur more commonly than had been recognized and show phenotypic and genotypic heterogeneity. Their study provides the opportunity to learn more about the role of the peroxisome in normal brain function and development.

On the role of peroxisomes in the metabolism of lipids--evidence from studies on mammalian tissues in vivo.

Recent investigations into the role of peroxisomes in mammalian lipid metabolism have employed double isotope methodologies to examine the influence of peroxisomal agents on lipid turnover in the liver and extra hepatic tissues of the living animal. The action of these agents, all of which caused extensive changes in the flux of lipid metabolism in the treated animals, may best be viewed in relation to their effects on the common pathway of fatty acid oxidation in peroxisomes. Clofibrate, for example, acts through induction of peroxisomal oxidases and catalase; glycolate and ethanol through activation of this pathway; and aminotriazole and allylisopropylacetamide through inhibition of the catalase step in the sequence. The data from these studies provide support for the concept of an important contributory and regulatory role of peroxisomes in relation to the overall balance of lipid metabolism, and emphasize that these organelles play a significant role in the oxidation of common fatty acids, as well as a potential for the elimination of fatty acids that are poorly oxidized by mitochondria. Additionally, the data raise intriguing questions on the extension of peroxisomal influence to include phospholipid metabolism and the substantial degree of inter-tissue communication which is involved in the balance of lipid metabolism in the whole animal.

Lipid Metabolism in Mammals. Volume 1 and 2.Fred Snyder

<i>Lipid Metabolism in Mammals. Volume 1 and 2.</i>Fred Snyder

Lipid Metabolism in Mammals, Volume 2

Lipid Metabolism in Mammals, Volume 2

Effect of heparin on the kinetics of guinea pig and human postheparin lipase activity.

SummaryLipoprotein lipase plays a major role in mammalian lipid metabolism. It is the key enzyme for hydrolysis of triglycerides contained in very-low-density lipoproteins (VLDL) and chylomicrons. Triglyceride hydrolysis by this enzyme requires that peptides be present on the chylomicron and VLDL surfaces. When artificial triglyceride emulsions are used in a postheparin lipase system, triglyceride hydrolysis depends upon emulsion activation by these same peptides. When triglyceride concentration is held constant in the assay system, an increase in high density lipoproteins or VLDL produces a Michaelis-Menten hyperbolic increase in lipase activity. Previous work has suggested that heparin added to the assay system modifies the kinetics of interaction of postheparin lipase with its triglyceride substrate. The result is a sigmoid substrate saturation curve, with heparin apparently acting as an allosteric modifier of the lipase. This study supports the role of heparin as an allosteric modifier of postheparin ...

The effects of a plant-growth retardant, phosfon, on mammalian lipid metabolism in vivo

Intraperitoneal injection of the plant-growth retardant, Phosfon, significantly stimulates the incorporation in vivo of radioactive acetate into cholesterol in several organs of the mature male rat. Amo 1618, another plant-growth retardant, does not produce this response.

Comparison of serum total lipid during cold exposure in hibernating and non-hibernating mammals.

LIPID metabolism in mammals has been widely investigated with reference to cold acclimation and hibernation. In this laboratory, work on lipid metabolism is hamsters during cold exposure and hibernation has been focused on rates of lipid synthesis or utilization in tissues in vitro. This communication is concerned with measurement of the circulating total lipid in the intact animal. Analyses were performed on blood from, cold-exposed and hibernating golden hamsters (Mesocricetus auratus). A similar series of analyses were performed on white Wistar rats to provide comparison with a non-hibernating mammal. Blood samples were taken from animals kept at room temperature as control, and kept in the cold (5° ± 1° C) for 48 h, 3 weeks or 6–8 weeks. Samples were also taken from hamsters in hibernation, arousing from hibernation (cheek pouch temperature 18°–20° C) and immediately after the animals regained full consciousness and normal temperature. The animals were stunned by a sharp blow on the head, except those in hibernation. Approximately 2 c.c. of blood were withdrawn by cardiac puncture. After removal of the needle the syringe was discharged into a centrifuge tube and the sample allowed to clot. The samples were centrifuged and the serum removed by suction. The serum was then analysed for total lipid by a colorimetric method employing Sudan black B, described by Swahn1. The samples were compared with a standard of corn oil in ethanol. The concentrations were calculated as mg total lipid/100 ml. of serum. The results are shown in Fig. 1.

Study of the cellular action of drugs with protozoa. II. Comparative study of the inhibitions induced by hypocholesteremic agents on Ochromonas danica multiplication; annulment by oleic acid.

SYNOPSIS. Several hypocholesteremic agents inhibit multiplication of Ochromonas danica, Euglena grucilis, and Tetrahy‐menu pyriformis. Inhibiting O. danica are benzmalecene, triparanol, diazacholestanol. 3‐β‐(2‐diethylaminoethoxy) androst‐5‐en‐17‐one, α‐phenylbutyric acid, β‐diethylaminoethyldiphenyl‐n‐propylacetate (SKF‐525A) 2,2‐diphenyl‐1‐ (β‐dimethylaminoethoxy)pentane (SKF‐3301A), trans‐3‐(p‐chlorophenyl)‐2‐[p‐(2‐diethylaminoethoxy)phenyll‐2‐pentenenitrile (P‐3429), frans‐3‐[p‐(2‐diethylaminoethoxy) phenyll‐2‐pentenenitrile (P‐3013), vanadyl SO† and L‐thyroxine but not nicotinic acid, 3‐pyridylacetic acid, 3‐pyridylacetamide, nicotinuric acid, p‐aminosalicylic acid. tris‐(2‐dimethylaminoethyl) phosphate (SKF‐7732‐A3), tris‐ (2‐diethylaminoethyl) phosphate (SKF‐7997‐A3), 2,3‐bis[p‐(2‐diethylaminoethoxy) phenyl] acrylonitrile bis (P‐2681).Inhibition of Ochromonas multiplication by benzmalecene, triparanol, diazacholestanol, SKF‐525A. SKF‐3301A, P‐3013, and P‐3429 was annulled by lecithin, TEM, and specifiically by oleic acid. The following compounds equimolar with oleic acid were ineffective: stearic, palmitic, myristic, and lauric acids; cholesterol, stigmasterol, ergosterol, lanosterol, and β‐sitosterol; mevalonic acid lactone, geranyl acetate, farnesol, and squalene.Evidence is presented for the usefulness of protozoa as detectors of toxicity and possible harmful side effects of inhibitors of lipid metabolism in mammals.