Selective Inhibition of Cholesterol Biosynthesis in Brain Cells by Squalestatin 1

Abstract: The effect of squalestatin 1 (SQ) on squalene synthase and other enzymes utilizing farnesyl pyrophosphate (F-P-P) as substrate was evaluated by in vitro enzymological and in vivo metabolic labeling experiments to determine if the drug selectively inhibited cholesterol biosynthesis in brain cells. Direct in vitro enzyme studies with membrane fractions from primary cultures of embryonic rat brain (IC₅₀ = 37 n M ), pig brain (IC₅₀ = 21 n M ), and C6 glioma cells (IC₅₀ = 35 n M ) demonstrated that SQ potently inhibited squalene synthase activity but had no effect on the long-chain cis -isoprenyltransferase catalyzing the conversion of F-P-P to polyprenyl pyrophosphate (Poly-P-P), the precursor of dolichyl phosphate (Dol-P). SQ also had no effect on F-P-P synthase; the conversion of [³H]F-P-P to geranylgeranyl pyrophosphate (GG-P-P) catalyzed by partially purified GG-P-P synthase from bovine brain; the enzymatic farnesylation of recombinant H-p21 ^ras by rat brain farnesyltransferase; or the enzymatic geranylgeranylation of recombinant Rab1A, catalyzed by rat brain geranylgeranyltransferase. Consistent with SQ selectively blocking the synthesis of squalene, when C6 glial cells were metabolically labeled with [³H]mevalonolactone, the drug inhibited the incorporation of the labeled precursor into squalene and cholesterol (IC₅₀ = 3–5 µ M ) but either had no effect or slightly stimulated the labeling of Dol-P, ubiquinone (CoQ), and isoprenylated proteins. These results indicate that SQ blocks cholesterol biosynthesis in brain cells by selectively inhibiting squalene synthase. Thus, SQ provides a useful tool for evaluating the obligatory requirement for de novo cholesterol biosynthesis in neurobiological processes without interfering with other critical reactions involving F-P-P.     

Brain dolichyl pyrophosphate phosphatase. Solubilization, characterization, and differentiation from dolichyl monophosphate phosphatase activity

Dolichyl [beta-32P]pyrophosphate ([beta-32P]Dol-P-P) has been prepared chemically to study Dol-P-P phosphatase in calf brain. Calf brain microsomes catalyze the enzymatic release of 32Pi from exogenous [beta-32P]Dol-P-P by a bacitracin-sensitive reaction. [32P]Pyrophosphate was not detected with the water-soluble product even when 1 mM sodium pyrophosphate was added to impede pyrophosphatase activity. A substantial fraction of the Dol-P-P phosphatase activity can be solubilized by treating brain microsomes with 3% Triton X-100. The detergent extracts catalyze the enzymatic release of 32Pi from [beta-32P]Dol-P-P and the conversion of [14C]undecaprenyl pyrophosphate to [14C]undecaprenyl monophosphate. The solubilized Dol-P-P phosphatase activity: 1) is optimal at neutral pH; 2) is inhibited by Mn2+ and stimulated by EDTA; 3) exhibits an apparent Km = 20 microM for Dol-P-P; 4) is competitively inhibited by undecaprenyl pyrophosphate, and 5) is blocked by bacitracin. Solubilized Dol-P-P phosphatase activity differs from Dol-P phosphatase activity present in the same detergent extracts with respect to: 1) thermolability at 50 degrees C, 2) effect of 20 mM EDTA, and 3) sensitivity to phosphate and fluoride ions. These studies describe the chemical synthesis of [beta-32P]Dol-P-P for use in a convenient assay of Dol-P-P phosphatase activity. A procedure for the solubilization of Dol-P-P phosphatase activity from microsomes is presented, and an enzymological comparison indicates that Dol-P-P and Dol-P phosphatase are separate enzymes in calf brain.     

Golgi-enriched membrane fractions from rat brain and liver contain long-chain polyisoprenyl pyrophosphate phosphatase activity.

The subcellular distribution of polyisoprenyl pyrophosphate phosphatase activity has been examined in rat brain by assaying the release of 32Pi from [beta-32P]dolichyl pyrophosphate (Dol-P-P) as described previously (Scher,M.G. and Waechter, C.J. (1984) J. Biol. Chem., 259, 14580-14585). The highest specific activities of Dol-P-P phosphatase in rat brain were found in the Golgi-enriched light microsomal, synaptic plasma membrane and heavy microsomal fractions. A comparative analysis of the distribution of galactosyltransferase and dolichol kinase reveals that Dol-P-P phosphatase activity co-fractionates with galactosyltransferase activity, and that the high level found in the Golgi-enriched fraction is not due to cross-contamination with heavy microsomes. When beta-labelled C95 Dol-P-P and the C95 allylic polyisoprenyl pyrophosphate (Poly-P-P) were compared as substrates for the Golgi-enriched light microsomal and heavy microsomal fractions, similar Km values were calculated for the two pyrophosphorylated substrates for each membrane fraction. Based on these kinetic analyses, the enzyme(s) catalysing this reaction do not distinguish between substrates containing saturated or allylic alpha-isoprene units. When Dol-P-P phosphatase activity was assessed in submicrosomal fractions obtained from rat liver by two separate procedures, the highest specific activity was also detected in the Golgi-enriched fraction. While the specific activities for Dol-P-P phosphatase and sialyltransferase were in the relative order of Golgi greater than smooth endoplasmic reticulum (ER) greater than rough ER, the relative order of dolichol kinase was rough ER greater than smooth ER greater than Golgi.(ABSTRACT TRUNCATED AT 250 WORDS)     

Biosynthesis of lipid-linked oligosaccharides in embryonic liver. Formation of dolichyl pyrophosphate N-acetylglucosaminyl intermediates

Liver microsomes from pig embryos synthesized dolichyl pyrophosphate N-acetylglucosamine and converted it to dolichyl pyrophosphate N,N'-diacetylchitobiose. N-acetylglucosaminyl transferase activity towards dolichol was about 2-fold greater in microsomes from embryonic liver than in microsomes from adult liver. A maximum level of conversion of dolichyl pyrophosphate N-acetylglucosamine to dolichyl pyrophosphate N,N'-diacetylchitobiose was achieved at 5 mM concentration of unlabelled UDP-N-acetylglucosamine, while this conversion was negligible at lower UDP-N-acetylglucosamine concentrations (0.1 and 0.5 mM). The level of dolichyl phosphate, assessed by the level of dolichyl pyrophosphate N-acetylglucosamine synthesis was 2-fold higher in microsomes from embryonic liver than that in microsomes from adult liver. Tunicamycin (1 microgram/ml) inhibited completely the formation of dolichyl pyrophosphate N-acetyl-glucosamine in embryonic liver microsomes, while the inhibitory effect of UMP (1 mM) was about 70%.     

On the feasibility of electron transfer to singlet oxygen from mitochondrial components

The incorporation of [¹⁴C]mannose from GDP-[¹⁴C]mannose into dolichyl mannosyl phosphate in rat liver microsomes showed a biphasic time-course; an initial rapid incorporation of mannose which ceased within 2 min and a much slower incorporation which continued for 30 min. In the presence of 0.18 mM (250 μg/ml) bacitracin, the rapid incorporation proceeded normally whereas the slow incorporation was inhibited by about 70%. Upon addition of dolichyl pyrophosphate, the microsomes catalyzed the dephosphorylation of the added compound which was also inhibited by bacitracin. The results, coupled with several other observations, suggest that the rapid reaction represents the transfer of mannose to endogenous dolichyl phosphate whereas the bacitracin-sensitive, slow reaction represents a more complex process in which the enzymatic dephosphorylation of dolichyl pyrophosphate is involved as a rate-limiting step.     

Characterization of the Saccharomyces cerevisiae cis-prenyltransferase required for dolichyl phosphate biosynthesis

The prenyltransferase involved in the biosynthesis of dolichyl phosphate has been characterized in Saccharomyces cerevisiae. Although the enzyme is predominantly membrane-bound, a significant percentage was found in the soluble fraction. The prenyltransferase preferentially utilizes farnesyl pyrophosphate as the allylic substrate and isopentenyl pyrophosphate as cosubstrate with half-maximal velocities obtained at 25 and 6.7 microM, respectively. The enzymatic activity is sensitive to sulfhydryl reagents and is inhibited by all detergents tested, except 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate at concentrations less than 5 mM. The product of the reaction has been characterized as an alpha-unsaturated polyprenyl pyrophosphate, containing 12-15 isoprene units, approximately two isoprene units shorter than the endogenous yeast dolichyl phosphate. The stereochemistry of addition of isoprene units by the prenyltransferase was shown to be cis by a comparison of the HPLC retention time for a pentadecaprenyl phosphate derived from the in vitro reaction product with that for an authentic mixture of alpha-cis- and alpha-trans-pentadecaprenyl phosphates.     

Pyrroline-5-carboxylate synthesis from glutamate by rat intestinal mucosa

The mitochondria of rat intestinal mucosa were found to have an enzymatic activity that converts radioactive glutamate to pyrroline-5-carboxylate (P5C) in the presence of ATP, NADPH, and MgCl2. The product of this enzyme was identified as P5C by the fact that it was converted to proline by chemical reduction with NaBH4 or by enzymatic reduction with NADH in the presence of purified yeast P5C reductase. The product was demonstrated to be P5C rather than pyrroline-2-carboxylate by thin layer chromatography. The presence of the activity in mitochondria prepared from intestinal mucosa of germ-free rats proved that this activity is of mammalian origin. Omission of either ATP, NADPH, or MgCl2 from the reaction mixture resulted in little or no activity. The optimal pH appeared to be about 7.0 under the conditions used. Substrate saturation curves in the presence of an ATP and an NADPH regeneration system gave apparent Km values of 2.5 mM for glutamate, 0.19 mM for ATP, and 6.5 microM for NADPH in the presence of 20 mM MgCl2. The mitochondrial preparation usually produced P5C at a rate of 1.2 to 1.6 nmol/mg/min at 20 degrees C when incubated with 1 mM glutamate, 3 mM ATP, 0.2 mM NADPH, and 20 mM MgCl2.     

The assembly of lipid-linked oligosaccharides in plant and animal membranes

Membrane preparations from growing regions of pea stems and activelydividing mouse L-cells form lipid-linked saccharides from GDP-mannose and UDP-N-acetylglucosamine. These lipids have properties which are consistent with those of mono-and di-phosphoryl polyisoprenyl derivatives. In experiments using plant membranes, the monophosphoryl derivative labeled with GDP-(¹⁴C) mannose contains mannose only, while the diphosphoryl derivative labeled with the same nucleotide sugar is heterogeneous, containing oligosaccharides corresponding to mannosaccharides of 5, 7, and 9-12 residues. Only the diphosphoryl polyisoprenyl derivatives are labeled with UDP-(¹⁴C)glucosamine and these contain predominantly chitobiose and N-acetylglucosamine itself. Unlabeled GDP-mannose added after UDP-N-acetyl (¹⁴C)glucosamine results in the formation of higher lipid-linked oligosaccharides which are apparently the same as those which are labeled with GDP-(¹⁴C)mannose alone. Incubation of the membranes with GDP-(¹⁴C)mannose in the presence of Mn²⁺, unlabeled UDP-glucose or unlabeled UDP-N-acetylglucosamine results in marked changes in the accumulation of both the polyisoprenyl monophosphoryl mannose and polyisoprenyl diphosphoryl oligosaccharides. Animal cell membranes synthesise lipid-linked oligosaccharides when incubated with UDP-N-acetylglucosamine and GDP-mannose. These oligosaccharides are similar in size to those synthesised by the plant membranes but their formation is more efficient. The potential roles of these compounds in glycoprotein biosynthesis in both plant and animal tissues is discussed.     

Transfer to proteins across membranes. II. Reconstitution of functional rough microsomes from heterologous components

The data presented in this paper demonstrate that native small ribosomal subunits from reticulocytes (containing initiation factors) and large ribosomal subunits derived from free polysomes of reticulocytes by the puromycin-KCl procedures can function with stripped microsomes derived from dog pancreas rough microsomes in a protein-synthesizing system in vitro in response to added IgG light chain mRNA so as to segregate the translation product in a proteolysis- resistant space. No such segregation took place for the translation product of globin mRNA. In addition to their ability to segregate the translation product of a specific heterologous mRNA, native dog pancreas rough microsomes as well as derived stripped microsomes were able to proteolytically process the larger, primary translation product in an apparently correct manner, as evidenced by the identical mol wt of the segregated translation product and the authentic secreted light chain. Segregation as well as proteolytic processing by native and stripped microsomes occurred only during ongoing translation but not after completion of translation. Attempts to solubilize the proteolytic processing activity, presumably localized in the microsomal membrane by detergent treatment, and to achieve proteolytic processing of the completed light chain precursor protein failed. Taken together, these results establish unequivocally that the information for segregation of a translation product is encoded in the mRNA itself, not in the protein- synthesizing apparatus; this provides strong evidence in support of the signal hypothesis.     

Enzymatic dephosphorylation of endogenous and exogenous dolichyl monophosphate by calf brain membranes

Calf brain membranes have been shown to enzymatically dephosphorylate endogenous and partially purified, exogenous dolichyl [³²P]monophosphate. The properties and specificity of the dolichyl monophosphatase activity have been studied by following the release of [³²P]phosphate from exogenous dolichyl [³²P]monophosphate added in a dispersion with Triton X-100. The calf brain phosphatase (1) is inhibited by Mn²⁺, Mg²⁺, Ca²⁺, fluoride, and phosphate; (2) exhibits a neutral pH optimum; and (3) has an apparent K_m of 200 μm for dolichyl monophosphate. Dolichyl monophosphatase activity can be distinguished from phosphatidate phosphatase on the basis of their responses to fluoride and phosphate. Based on differential thermolability and the effects of divalent cations and EDTA, the calf brain dolichyl monophosphatase can also be discriminated from the general phosphatase activity assayed with p-nitrophenyl phosphate. Dolichyl monophosphatase activity can be solubilized by treating microsomes with Triton X-100. The enzymatic dephosphorylation of exogenous dolichyl [³²P]monophosphate catalyzed by particulate and detergent-solubilized preparations is negligibly affected by equimolar concentrations of ATP and an assortment of phosphomonoesters, including phosphatidic acid and hexadecyl phosphate. A reduction of approximately 40% in dolichyl monophosphatase activity is observed in the presence of equimolar amounts of retinyl monophosphate. Overall, these results represent good evidence for the presence of a neutral polyisoprenyl monophosphatase in central nervous tissue.     

Biosynthesis of dolichyl pyrophosphate N-acetylglucosaminyl intermediates in liver microsomes from hibernating ground squirrels (Citellus citellus L.)

Dolichyl pyrophosphate N-acetyl[14C]glucosamine was synthesized after incubation of liver microsomes from hibernating ground squirrels with UDP-N-acetyl[14C )glucosamine. The radioactivity of glycolipid formed by liver microsomes from hibernating ground squirrels was about 2-fold greater than by liver microsomes from active animals. Addition of exogenous dolichyl phosphate to the incubation mixture increased the formation of dolichyl pyrophosphate N-acetyl[14C]glucosamine by microsomes from both active and hibernating ground squirrels about 6 times. Liver microsomes from hibernating ground squirrels converted dolichyl pyrophosphate N-acetyl[14C]glucosamine into dolichyl pyrophosphate N,N'-diacetyl[14C]chitobiose in the presence of unlabelled UDP-N-acetylglucosamine. This conversion was maximal at 1.0 M concentration of unlabelled UDP-N-acetylglucosamine. The level of dolichyl phosphate assessed by the level of dolichyl pyrophosphate N-acetylglucosamine formation was nearly 2 times greater in liver microsomes from hibernating ground squirrels than from active animals.     

The mechanism and regulation of dolichyl phosphate biosynthesis in rat liver

Rat liver slices were pulse labeled for 6 min with [3H]mevalonolactone and then chased for 90 min with unlabeled mevalonolactone in order to study the mechanism of dolichyl phosphate biosynthesis. The cholesterol pathway was also monitored and served to verify the pulse-chase. Under conditions in which radioactivity in the methyl sterol fraction chased to cholesterol, radioactivity in alpha-unsaturated polyprenyl (pyro)-phosphate chased almost exclusively into dolichyl (pyro)phosphate. Lesser amounts of radioactivity appeared in alpha-unsaturated polyprenol and dolichol, and neither exhibited significant decline after 90 min of incubation. The relative rates of cholesterol versus dolichyl phosphate biosynthesis were studied in rat liver under four different nutritional conditions using labeled acetate, while the absolute rates of cholesterol synthesis were determined using 3H2O. From these determinations, the absolute rates of dolichyl phosphate synthesis were calculated. The absolute rates of cholesterol synthesis were found to vary 42-fold while the absolute rates of dolichyl phosphate synthesis were unchanged. To determine the basis for this effect, the rates of synthesis of cholesterol and dolichyl phosphate were quantitated as a function of [3H]mevalonolactone concentration. Plots of nanomoles incorporated into the two lipids were nearly parallel, yielding Km values on the order of 1 mM. In addition, increasing concentrations of mevinolin yielded parallel inhibition of incorporation of [3H]acetate into cholesterol and dolichyl phosphate. The specific activity of squalene synthase in liver microsomes from rats having the highest rate of cholesterol synthesis was only 2-fold greater than in microsomes from rats having the lowest rate. Taken together, the results suggest that the maintenance of constant dolichyl phosphate synthesis under conditions of enhanced cholesterogenesis is not due to saturation of the dolichyl phosphate pathway by either farnesyl pyrophosphate or isopentenyl pyrophosphate but coordinate regulation of hydroxymethylglutaryl-CoA reductase and a reaction on the pathway from farnesyl pyrophosphate to cholesterol.     

Formation of lipid-linked sugar compounds in Halobacterium salinarium. Presumed intermediates in glycoprotein synthesis.

The ability of bacitracin to inhibit the growth of Halobacterium salinarium suggested that glycosylation of the major envelope component, a high molecular weight glycoprotein, might occur via a pathway involving lipid intermediates. This report demonstrates that the cells have enzymatic activities for formation of lipid-linked sugar compounds having the expected properties of such intermediates. Whole cell homogenate catalyzed the transfer of sugar from UDP-glucose, GDP-mannose, and UDP-N-acetyglucosamine to endogenous lipid acceptors. Two lipid products were formed from UDP-glucose, two from GDP-mannose, and one from UDP-N-acetylglucosamine. Characterization of the partially purified lipids by ion exchange chromatography, thin layer chromatography, and mild acid and base hydrolysis showed the major product in each case to have the properties expected for polyisoprenyl phosphoglucose, polyisoprenyl phosphomannose, and polyisoprenyl pyrophospho-N-acetylglucosamine. Estimates of chain length by thin layer chromatography indicate that the lipid has 11 to 12 isoprene identity as a C55-60-polyisoprenyl pyrophospho-N-acetylglucosamine. The N-acetylglucosamine transferase, present in cell envelope preparations, was partially characterized. The enzyme was found to be extremely halophilic, specifically requiring a high concentration of KCl. Optimum activity was obtained at 4 m KCl and partial substitution of K+ by Na+ resulted in a decrease in activity.     

Thiamine pyrophosphate requirement for o-succinylbenzoic acid synthesis in Escherichia coli and evidence for an intermediate.

Cell-free extracts of various strains of Escherichia coli synthesize the menaquinone biosynthetic intermediate o-succinylbenzoic acid (OSB) when supplied with chorismic acid, 2-ketoglutaric acid, and thiamine pyrophosphate (TPP). To assay for OSB synthesis, 2-[U-14C]ketoglutaric acid was used as substrate, and the synthesized OSB was examined by radiogas chromatography (as the dimethyl ester). [U-14C]Shikimic acid also gave rise to radioactive OSB if the cofactors necessary for enzymatic conversion to chorismic acid were added. Use of 2-[1-14C]ketoglutaric acid does not give rise to labeled OSB. In the absence of TPP during the incubations, OSB synthesis was much reduced; these observations are consistent with the proposed role for the succinic semialdehyde-TPP anion as the reagent adding to chorismic acid. Extracts of cells from menC and menD mutants did not form OSB separately, but did so in combination. There was evidence for formation of a product, X, by extracts of a menC mutant incubated with chorismic acid, TPP, and 2-ketoglutaric acid; X was converted to OSB by extracts of a menD mutant. It appears that the intermediate, X, is formed by one gene product and converted to OSB by the second gene product.     

Partial purification and properties of prenyltransferase from Pisum sativum

Prenyltransferase (EC 2.5.1.1; assayed as farnesyl pyrophosphate synthetase)was purified 106-fold from an homogenate of 3-day-old seedlings of Pisum sativum. Some of the properties of the purified enzyme were determined and these differed in several significant respects from those reported for preparations from other sources, e.g. the apparent MW was 96000 ± 4000 and the preparation could be dissociated into two subunits of MW 45000 ± 3000. The total activity of the extractable enzyme went through a sharp maximum (in the range 1 to 28 days) 3 days after germination. Farnesyl pyrophosphate was formed in cell-free extracts of peas from either isopentenyl pyrophosphate alone, or this together with geranyl pyrophosphate (optimum yields 1.2 and 10% respectively). Use of [1-¹⁴C]- and [4-¹⁴C]-isopentenyl pyrophosphates as the sole substrates and degradation of the products showed that the crude extracts contained a pool of the biogenetic equivalent of 3,3-dimethylallyl pyrophosphate. No analogous pool of geranyl pyrophosphate could be detected.     

Feedback inhibition of polyisoprenyl pyrophosphate synthesis from mevalonate in vitro. Implications for protein prenylation.

The prenylation of proteins utilizes the polyisoprenyl pyrophosphates (FPP) and geranylgeranyl pyrophosphate (GGPP) as prenyl donors. These polyisoprenoids are also precursors to ubiquinone and dolichol synthesis. We have previously described the geranylgeranylation of rab 1b from labeled mevalonate in rabbit reticulocyte lysates (Khosravi-Far, R., Lutz, R. J., Cox, A. D., Conroy, L., Bourne, J. R., Sinensky, M., Balch, W. E., Buss, J. C., and Der, C. J. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 6264-6268). We now directly demonstrate the incorporation of mevalonate into FPP and GGPP in rabbit reticulocyte cytosol. High pressure liquid chromatography analysis reveals that only all-trans-E,E,E-GGPP, the prenyl donor for in vivo protein geranylgeranylation, is synthesized. Incubations with recombinant H-ras and rab1b result in an increased synthesis of farnesyl and geranylgeranyl derivatives, respectively. The increase is wholly accounted for by protein-incorporated polyisoprenoids with no change in the polyisoprenyl pyrophosphate pools. Further, GGPP inhibits its own synthesis, without affecting FPP synthesis, with half-maximal inhibition at approximately 3 microM GGPP. Inhibition of FPP synthesis by the inhibition of isopentenyl isomerase causes a dramatic increase in isopentenyl pyrophosphate synthesis. FPP also inhibits conversion of mevalonate into FPP. These findings indicate that these polyisoprenyl pyrophosphates can down-regulate their own synthesis in vitro, and this regulation may control the levels of these polyisoprenoids in vivo.     

Alpha-hydroxylation of lignoceroyl-CoA in rat brain microsomes: involvement of NADPH-cytochrome c reductase and topical distribution

1. The enzymatic mechanism of the alpha-hydroxylation of lignoceroyl-CoA, an intermediate in the synthesis of hydroxyceramide, was studied. In the presence of NADPH, sphingosine and microsomes from 20-day-old rat brain, 14C from [1-14C]lignoceroyl-CoA was incorporated into hydroxyceramide. 2. The alpha-hydroxylation of lignoceroyl-CoA in rat brain microsomes was strongly inhibited by a rabbit anti-immunoglobulin G which was prepared against rat liver microsomal NADPH-cytochrome c reductase. However, anti-immunoglobulin G against cytochrome b5 did not inhibit the alpha-hydroxylase activity. 3. The alpha-hydroxylation activity was more sensitive to trypsin treatment than was NADPH-cytochrome c reductase in rat brain microsomes. This indicates that either alpha-hydroxylase itself or an unknown factor essential in alpha-hydroxylation is highly exposed to the surface of brain microsomes.     

Biosynthesis of monoterpenes: demonstration of a geranyl pyrophosphate:(-)-bornyl pyrophosphate cyclase in soluble enzyme preparations from tansy (Tanacetum vulgare)

Tansy (Tanacetum vulgare L.) produces an essential oil containing the optically pure monoterpene ketone, (-)-camphor, as a major constituent. A soluble enzyme preparation from immature leaves of this plant converts the acyclic precursor [1-3H]geranyl pyrophosphate to the bicyclic monoterpene alcohol borneol in the presence of MgCl2, and oxidizes a portion of the borneol to camphor in the presence of a pyridine nucleotide. The identity of the major biosynthetic product as borneol was confirmed by chemical oxidation to camphor and crystallization of the derived oxime to constant specific radioactivity. The stereochemistry of the borneol was verified as the (-)-(1S,4S) isomer by oxidation to camphor, conversion to the corresponding ketal with D-(-)-2,3-butanediol, and separation of diastereoisomers by radio-gas-liquid chromatography. When enzyme reaction mixtures were treated with a mixture of acid phosphatase and apyrase, following an initial ether extraction of labeled borneol, additional quantities of borneol were generated, indicating the presence of a phosphorylated derivative of borneol. This water-soluble metabolite was prepared by large-scale enzyme incubations with [1-3H]geranyl pyrophosphate (plus phosphatase inhibitor), and the identity of the initial cyclization product was established as (-)-bornyl pyrophosphate by direct ion-exchange chromatographic analysis and enzymatic hydrolysis. The pathway for the formation of (-)-(1S,4S)-camphor was therefore identical to that previously demonstrated for the (+)-(1R,4R) isomer, involving cyclization of geranyl pyrophosphate to bornyl pyrophosphate, hydrolysis of this intermediate to borneol, and oxidation of the alcohol to the ketone. The labeling pattern of the product derived from [1-3H2, U-14C]geranyl pyrophosphate was determined by oxidation of the biosynthetic borneol to camphor and selective removal of tritium by exchange of the alpha hydrogens at C3 of the ketone. This labeling pattern was identical to that observed previously for the (+) isomer, suggesting the same mechanism of cyclization, but of opposite enantiospecificity. Some properties of the antipodal (+)- and (-)-bornyl pyrophosphate cyclases were compared.     

Cytidine 3',5'-monophosphate (cyclic CMP) formation by homogenates of mouse liver.

Cyclic CMP³ has been identified as a product of the reaction between mouse liver homogenate, CTP and Mn²⁺ at neutral pH and 37°. This reaction appears to be enzymatic in character in that product formation is pH-, temperature-, time-, and substrate-dependent, and is inhibited by boiling the homogenate. Cyclic CMP formation is enhanced with 0.3 mM Mn²⁺ or Fe²⁺ and inhibited with 3 mM Mn²⁺ or detergents. Cyclic CMP was identified as one of the reaction products by comparison with authentic compound in several systems including: chromatography on neutral alumina columns, Dowex 1-formate columns, polyethyleneimine cellulose columns and thin layer plates; crystallization to constant specific activity; radioimmunoassay.