Mechanism of inhibition of yeast squalene synthase by substrate analog inhibitors

Squalene synthase catalyzes the reductive condensation of two identical substrate molecules, farnesyl diphosphate, to the hydrocarbon squalene via an obligatory intermediate, presqualene pyrophosphate. Since the kinetic mechanism of the transformation is sequential, two substrate binding pockets that recognize the same molecule must exist in the enzyme active site. This raises the possibility of a choice of binding pockets for inhibitors that are designed as substrate or reaction intermediate analogs and thus may provide some information on the mechanism of differentiation of the two identical molecules. In this report, we have investigated the mechanism of inhibition of a series of farnesyl diphosphate analog inhibitors. The inhibitors fall into two categories. One class of compounds binds to free enzyme as well as the enzyme substrate complex, and the binding is refractory to the concentration of the substrate. The second class binds only to the free enzyme, and its binding is significantly modulated by the substrate concentration. Very modest structural changes in the compounds appear to dictate which class of inhibitor any compound may fall into. The significance of these observations with respect to the mechanism of the enzyme are discussed.

Glucolipsin A and B, two new glucokinase activators produced by Streptomyces purpurogeniscleroticus and Nocardia vaccinii

During the screening of the natural products for their ability to increase the activity of glucokinase by relieving inhibition by long chain fatty acyl CoA esters (FAC), two novel compounds, glucolipsin A (1) and B (2) were isolated from the butanol extracts of Streptomyces purpurogeniscleroticus WC71634 and Nocardia vaccinii WC65712, respectively. The structures of these two compounds were established by spectroscopic methods and chemical degradation. Glucolipsin A (1) and B (2) relieved the inhibition of glucokinase by FAC with RC50 values of 5.4 and 4.6 microM.

Glucokinase regulatory protein may interact with glucokinase in the hepatocyte nucleus

Glucokinase (GK) plays a central role in the sensing of glucose in pancreatic beta-cells and parenchymal cells of the liver. Glucokinase regulatory protein is a physiological inhibitor of GK in the liver. To understand the role of the interaction of these two proteins in glucose sensing, we carried out a series of experiments to localize the protein in the liver cell. The regulatory protein was found to be present mainly in the nucleus of the cell under a variety of conditions that mimicked the glucose status of the fed and fasted state. GK was localized in the nucleus when the cells were exposed to low glucose concentrations. At higher glucose concentrations or in the presence of low concentrations of fructose, GK translocated to the cytoplasm. The effect of fructose was more robust and rapid than the effect of high glucose concentrations. Furthermore, the effect of fructose and high glucose on the translocation of GK from the nucleus could be partially reversed by glucagon. This unusual localization and behavior suggests a role for GK and its regulatory protein in hepatic energy metabolism that may be broader than glucose phosphorylation.

Heterologous expression and characterization of rat liver glucokinase regulatory protein

Glucokinase is a critical component of the physiological glucose sensor found in cell types that are responsive to changes in plasma glucose levels. The acute regulation of glucokinase activity has been shown to occur via a regulatory protein found in liver parenchymal cells (Van Schaftingen E, Detheux M, Da Cunha MV. Faseb J 8:414-419, 1994). The action of this protein is modulated by phosphate esters of fructose. In the presence of fructose-6-phosphate, the protein inhibits glucokinase in an allosteric competitive manner, while fructose-1-phosphate reverses this inhibition. A cDNA potentially encoding the rat liver regulatory protein has been cloned, but its identity is uncertain because of the small amounts of soluble protein obtained by expression in bacteria. We report the heterologous expression of the regulatory protein in Escherichia coli and its purification to homogeneity and high specific activity in a single chromatographic step. The properties of this recombinant protein are very similar to those of the liver protein. Direct demonstration of the binding of the recombinant protein to glucokinase has been obtained in vitro using coprecipitation experiments and in vivo, using the yeast two-hybrid system. These studies establish that the protein encoded by the cDNA is identical to the glucokinase regulatory protein and also validate tools with which to carry out structure-function studies on the interaction of the regulatory protein with glucokinase.

Yeast squalene synthase. A mechanism for addition of substrates and activation by NADPH

Squalene synthase catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to give presqualene diphosphate (PSPP) and the subsequent reductive rearrangement of PSPP to squalene. Previous studies of the mechanism of addition of FPP to the enzyme have led to conflicting interpretations of initial velocity measurements (Beytia, E., Qureshi, A. A., and Porter, J.W. (1973) J. Biol. Chem. 248, 1856-1867; Agnew, W.S., and Popjak, G. (1978) J. Biol. Chem. 253, 4566-4573). Initial velocities for synthesis of PSPP and squalene were measured over a wider range of FPP and NADPH concentrations than previously reported, using a soluble form of recombinant enzyme. In the absence of NADPH, PSPP formation was activated by FPP at concentrations above approximately 0.5 microM. At fixed levels of NADPH, the dependence of initial rates of PSPP and squalene synthesis on FPP concentrations indicated that the C15 substrate added by a sequential mechanism. In addition, NADPH stimulated synthesis of PSPP by 40-fold at saturating levels of the cofactor. This stimulation is, at least in part, by reduction of PSPP to squalene.

Molecular cloning of human mevalonate kinase and identification of a missense mutation in the genetic disease mevalonic aciduria

Mevalonic aciduria is the first proposed inherited disorder of the cholesterol/isoprene biosynthetic pathway in humans, and it is presumed to be caused by a mutation in the gene coding for mevalonate kinase. To elucidate the molecular basis of this inherited disorder, a 2.0-kilobase human mevalonate kinase cDNA clone was isolated and sequenced. The 1188-base pair open reading frame coded for a 396-amino acid polypeptide with a deduced M(r) of 42,450. The predicted protein sequence displayed similarity to those of galactokinase and the yeast RAR1 protein, indicating that they may belong to a common gene family. Southern hybridization studies demonstrated that the mevalonate kinase gene is located on human chromosome 12 and is a single copy gene. No major rearrangements were detected in the mevalonic aciduria subject. The relative size (2 kilobases) and amounts of human mevalonate kinase mRNA were not changed in mevalonic aciduria fibroblasts. Approximately half of the mevalonic aciduria cDNA clones encoding mevalonate kinase contained a single base substitution (A to C) in the coding region at nucleotide 902 that changed an asparagine residue to a threonine residue. The presence of this missense mutation was confirmed by polymerase chain reaction amplification and allele-specific hybridization of the genomic DNAs from the proband and the proband's father and brother. Similar analysis failed to detect this mutation in the proband's mother, seven normal subjects, or four additional mevalonic aciduria subjects, indicating that the mutation does not represent a common gene polymorphism. Functional analysis of the defect by transient expression confirmed that the mutation produced an enzyme with diminished activity. Our data suggest that the index case is a compound heterozygote for a mutation in the mevalonate kinase gene.

Effects of long-term administration of HMG-CoA reductase inhibitors on cholesterol synthesis in lens

The effects of long-term dosing with inhibitors of 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase on the rate of cholesterol biosynthesis were examined in the lens and liver of rats and hamsters. While both pravastatin and lovastatin inhibited incorporation of [14C]acetate into cholesterol in liver slices 2-4 hr after an oral dose, lovastatin, but not pravastatin, inhibited sterol synthesis in lens as well. At 24 hr after a single oral dose, cholesterol synthesis in livers from drug-treated animals was increased compared to controls. This induction of the cholesterol synthetic pathway was observed for both drugs in the liver but only for lovastatin in the lens. After 4 days of once-daily oral doses, synthesis in the lens was induced two to threefold by lovastatin but not by pravastatin. When the drug was included in the continuous diet for 4-5 days, lovastatin caused increases in cholesterol synthesis in the lens whereas lenses from pravastatin-treated animals were identical to controls. This was not a species-specific effect since a similar tissue selectivity was observed in the hamster. The increase in cholesterol synthesis in lenses observed in lovastatin-treated rats was accompanied by an increase in the activity of HMG-CoA reductase enzyme. These studies demonstrate that non-selective HMG-CoA reductase enzyme inhibitors can inhibit cholesterol synthesis in the lens, and following this inhibition a marked induction in the cholesterol biosynthetic pathway develops in the lens and this induction is associated with an increase in HMG-CoA reductase enzyme activity.

Tissue-selective acute effects of inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase on cholesterol biosynthesis in lens

Inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the key enzyme that regulates cholesterol synthesis, lower serum cholesterol by increasing the activity of low density lipoprotein (LDL) receptors in the liver. In rat liver slices, the dose-response curves for inhibition of [14C]acetate incorporation into cholesterol were similar for the active acid forms of lovastatin, simvastatin, and pravastatin. The calculated IC50 values were approximately 20-50 nM for all three drugs. Interest in possible extrahepatic effects of reductase inhibitors is based on recent findings that some inhibitors of HMG-CoA reductase, lovastatin and simvastatin, can cause cataracts in dogs at high doses. To evaluate the effects of these drugs on cholesterol synthesis in the lens, we developed a facile, reproducible ex vivo assay using lenses from weanling rats explanted to tissue culture medium. [14C]Acetate incorporation into cholesterol was proportional to time and to the number of lenses in the incubation and was completely eliminated by high concentrations of inhibitors of HMG-CoA reductase. At the same time, incorporation into free fatty acids was not inhibited. In marked contrast to the liver, the dose-response curve for pravastatin in lens was shifted two orders of magnitude to the right of the curves for lovastatin acid and simvastatin acid. The calculated IC50 values were 4.5 +/- 0.7 nM, 5.2 +/- 1.5 nM, and 469 +/- 42 nM for lovastatin acid, simvastatin acid, and pravastatin, respectively. Thus, while equally active in the liver, pravastatin was 100-fold less inhibitory in the lens compared to lovastatin and simvastatin. Similar selectivity was observed with rabbit lens. Following oral dosing, ex vivo inhibition of [14C]acetate incorporation into cholesterol in rat liver was similar for lovastatin and pravastatin, but cholesterol synthesis in lens was inhibited by lovastatin by as much as 70%. This inhibition was dose-dependent and no inhibition in lens was observed with pravastatin even at very high doses. This tissue-selective inhibition of sterol synthesis by pravastatin was likely due to the inability of pravastatin to enter the intact lens since pravastatin and lovastatin acid were equally effective inhibitors of HMG-CoA reductase enzyme activity in whole lens homogenates. We conclude that pravastatin is tissue-selective with respect to lens and liver in its ability to inhibit cholesterol synthesis.