Biosynthesis of penitrems and roquefortine by Penicillium crustosum.

Roquefortine and the penitrems were biosynthesised concurrently at an approximately equimolar rate by Penicillium crustosum after growth and sporulation. [14C]mevalonic acid was incorporated (15% efficiency) into the isoprenoid regions of the penitrem and roquefortine molecules to an extent consistent with their 6:1 molar ratio of isoprenoid components. [14C]penitrem A (specific activity, 3.4 X 10(2) mu Ci mmol-1) and 14C-penitrems B, C, and E readministered to young cultures were metabolically interconverted, indicating considerable metabolic flux, though generally directed towards penitrem A as the end product and suggesting a metabolic grid for the penitrem metabolites. Addition of bromide to the medium preferentially favored the production of bromo-analogs rather than the usual chloropenitrems.     

Frontalin: De novo biosynthesis of an aggregation pheromone component by Dendroctonus spp. bark beetles (Coleoptera: Scolytidae)

The pheromone component, frontalin (1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane) is thought to be formed in Dendroctonus spp. bark beetles through the cyclization of oxygenated 6-methyl-6-hepten-2-one (6-MHO). Unlike many of the isoprenoid pheromone components of bark beetles, there is no obvious immediate host conifer precursor for 6-MHO or frontalin. To elucidate the biosynthetic pathway of frontalin, juvenile hormone-treated male Dendroctonus jeffreyi were injected separately with [1-(14)C]acetate, [2-(14)C]mevalonolactone, [1-(14)C]isopentenol, [1-(14)C]:[1-(3)H]isopentenol, and [4,5-(3)H]leucine. Subsequently volatiles were collected on Porapak Q from these males and abdominal tissues were extracted. Radio-HPLC analyses of extracts from males injected with each radiolabeled substrate showed that radioactivity from the injected precursors eluted in a peak with a retention time that matches that of unlabeled frontalin. In all cases, HPLC fractions containing radiolabel that eluted at the same time as a frontalin standard were analyzed by GC-FID and GC-MS to confirm the presence of frontalin. In a separate study, male D. jeffreyi were injected with [1-(13)C]acetate and an abdominal tissue extract from these insects was analyzed by tandem gas chromatography-isotope ratio monitoring-mass spectrometry (GC-IRM-MS), which unequivocally showed incorporation of (13)C into frontalin. Because mevalonate is the key intermediate in the isoprenoid pathway, its incorporation (as mevalonolactone) into frontalin provides compelling evidence that the biosynthesis of frontalin involves that pathway in some form. In the experiment with [1-(14)C]:[1-(3)H]isopentenol, there was no significant difference in the mean percentage incorporation of either radioisotope into frontalin. This supports the role of the classical isoprenoid pathway, as tritium would be lost if only a hybrid pathway were involved. Confirming that de novo synthesis may be general to all Dendroctonus spp., (14)C-acetate was also incorporated into frontalin by females of D. rufipennis and D. simplex. A radiolabeled precursor/pathway inhibitor study showed that the fatty acid synthase inhibitor, 2-octynoic acid, increased (although not significantly) the mass of frontalin produced and significantly increased the percentage incorporation of radioactivity from [1-(14)C]acetate into frontalin. This suggests that as fatty acid biosynthesis is blocked, an increased amount of acetate is funneled into frontalin production via the isoprenoid pathway.     

The biosynthetic incorporation of the intact leucine skeleton into sterol by the trypanosomatid Leishmania mexicana

The amino acid leucine is efficiently used by the trypanosomatid Leishmania mexicana for sterol biosynthesis. The incubation of [2-(13)C]leucine with L. mexicana promastigotes in the presence of ketoconazole gave 14alpha-methylergosta-8,24(24(1))-3beta-ol as the major sterol, which was shown by mass spectrometry to contain up to six atoms of (13)C per molecule. (13)C NMR analysis of the 14alpha-methylergosta-8,24(24(1))-3beta-ol revealed that it was labeled in only six positions: C-2, C-6, C-11, C-12, C-16, and C-23. This established that the leucine skeleton is incorporated intact into the isoprenoid pathway leading to sterol; it is not converted first to acetyl-CoA, as in animals and plants, with utilization of the acetyl-CoA to regenerate 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). An inhibitor of HMG-CoA synthase (L-659,699) blocked the incorporation of [1-(14)C]acetate into sterol but had no inhibitory effect on [U-(14)C]leucine incorporation. The HMG-CoA reductase inhibitor lovastatin inhibited promastigote growth and [U-(14)C]leucine incorporation into sterol. The addition of unlabeled mevalonic acid (MVA) overcame the lovastatin inhibition of growth and also diluted the incorporation of [1-(14)C]leucine into sterol. These results are compatible with two routes by which the leucine skeleton may enter intact into the isoprenoid pathway. The catabolism of leucine could generate HMG-CoA that is then directly reduced to MVA for incorporation into sterol. Alternatively, a compound produced as an intermediate in leucine breakdown to HMG-CoA (e.g. dimethylcrotonyl-CoA) could be directly reduced to produce an isoprene alcohol followed by phosphorylation to enter the isoprenoid pathway post-MVA.     

Calditol tetraether lipids of the archaebacterium Sulfolobus solfataricus. Biosynthetic studies.

Lipids from the archaebacterium Sulfolobus solfataricus are based on 72-membered macrocyclic tetraethers made up from two C40 diol units differently cyclized and either two glycerol moieties or one glycerol moiety and a unique branched-chain nonitol named calditol (glycerodialkylnonitol tetraethers, GDNTs). To elucidate the biosynthesis of calditol and related tetraethers, labelled precursors, [U-14C,1(3)-3H]glycerol, [U-14C,2-3H]glycerol, D-[1-14C,6-3H]glucose, D-[6-14C,1-3H]glucose, D-[1-14C,2-3H]glucose, D-[1-14C,6-3H]fructose and D-[1-14C]galactose, were fed to S. solfataricus. Without regard to stereochemistry or phosphorylation, incorporation experiments provided evidence that the biosynthesis of calditol occurs via an aldolic condensation between dihydroxyacetone and fructose, through a 2-oxo derivative of calditol as an intermediate. The latter is in turn reduced and then alkylated to yield the GDNTs. The biogenetic origins of both glycerol and C40 isoprenoid moieties of GDNTs are also discussed.     

In vitro neuropharmacological evaluation of penitrem-induced tremorgenic syndromes: importance of the GABAergic system

The effects of the fungal neurotoxin penitrem A on the GABAergic and glutamatergic systems in rat brain were evaluated. Penitrem A inhibited binding of the GABA_A-receptor ligand [³H]TBOB to rat forebrain and cerebellar membrane preparations with IC₅₀ (half maximal inhibitory concentration) values of 11 and 9 μM, respectively. Furthermore, penitrem A caused a concentration-dependent increase of [³H]flunitrazepam and [³H]muscimol binding in rat forebrain, but not in cerebellar preparations. The stimulation of [³H]flunitrazepam binding by penitrem A was abolished by the addition of GABA. In cerebellar preparations, a different pharmacological profile was found, with penitrem A allosterically inhibiting [³H]TBOB binding by interacting with a bicuculline-sensitive site. Moreover, penitrem A inhibited the high affinity uptake of GABA and glutamate into cerebellar synaptosomes with IC₅₀ values of 20 and 47 μM, respectively. The toxin showed no effect on NMDA or AMPA glutamate receptor binding. In conclusion, our results suggest that penitrem A exerts region-specific effects in the brain, leading to positive modulation of GABA_A-receptor function in forebrain. Conversely, penitrem A may act as a bicuculline-like convulsant in cerebellum.     

Metabolic flux ratio analysis by parallel 13C labeling of isoprenoid biosynthesis in Rhodobacter sphaeroides

Metabolic engineering for increased isoprenoid production often benefits from the simultaneous expression of the two naturally available isoprenoid metabolic routes, namely the 2-methyl-D-erythritol 4-phosphate (MEP) pathway and the mevalonate (MVA) pathway. Quantification of the contribution of these pathways to the overall isoprenoid production can help to obtain a better understanding of the metabolism within a microbial cell factory. Such type of investigation can benefit from ¹³C metabolic flux ratio studies. Here, we designed a method based on parallel labeling experiments (PLEs), using [1-¹³C]- and [4-¹³C]glucose as tracers to quantify the metabolic flux ratios in the glycolytic and isoprenoid pathways. By just analyzing a reporter isoprenoid molecule and employing only four equations, we could describe the metabolism involved from substrate catabolism to product formation. These equations infer ¹³C atom incorporation into the universal isoprenoid building blocks, isopentenyl-pyrophosphate (IPP) and dimethylallyl-pyrophosphate (DMAPP). Therefore, this renders the method applicable to the study of any of isoprenoid of interest. As proof of principle, we applied it to study amorpha-4,11-diene biosynthesis in the bacterium Rhodobacter sphaeroides. We confirmed that in this species the Entner-Doudoroff pathway is the major pathway for glucose catabolism, while the Embden-Meyerhof-Parnas pathway contributes to a lesser extent. Additionally, we demonstrated that co-expression of the MEP and MVA pathways caused a mutual enhancement of their metabolic flux capacity. Surprisingly, we also observed that the isoprenoid flux ratio remains constant under exponential growth conditions, independently from the expression level of the MVA pathway. Apart from proposing and applying a tool for studying isoprenoid biosynthesis within a microbial cell factory, our work reveals important insights from the co-expression of MEP and MVA pathways, including the existence of a yet unclear interaction between them.     

Absorption and retinol equivalence of beta-carotene in humans is influenced by dietary vitamin A intake

The effect of vitamin A supplements on metabolic behavior of an oral tracer dose of [14C]beta-carotene was investigated in a longitudinal test-retest design in two adults. For the test, each subject ingested 1 nmol of [14C]beta-carotene (100 nCi) in an emulsified olive oil-banana drink. Total urine and stool were collected for up to 30 days; concentration-time patterns of [14C]beta-carotene, [14C]retinyl esters, and [14C]retinol were determined for 46 days. On Day 53, the subjects were placed on a daily vitamin A supplement (10000 IU/day), and a second dose of [14C]beta-carotene (retest) was given on Day 74. All 14C determinations were made using accelerator mass spectrometry. In both subjects, the vitamin A supplementation was associated with three main effects: 1). increased apparent absorption: test versus retest values rose from 57% to 74% (Subject 1) and from 52% to 75% (Subject 2); 2). an approximately 10-fold reduction in urinary excretion; and 3). a lower ratio of labeled retinyl ester/beta-carotene concentrations in the absorptive phase. The molar vitamin A value of the dose for the test was 0.62 mol (Subject 1) and 0.54 mol (Subject 2) vitamin A to 1 mol beta-carotene. Respective values for the retest were 0.85 and 0.74. These results show that while less cleavage of beta-carotene occurred due to vitamin A supplementation, higher absorption resulted in larger molar vitamin A values.     

RATE OF STEROL FORMATION BY RAT BRAIN GLIA AND NEURONS IN VITRO AND IN VIVO

The ability of 11-day-old rat glial and neuronal cells to biosynthesize sterol was studied as a function of time in vivo and in vitro. The in vitro experiments utilized [2-¹⁴C]mevalonic acid as precursor. Glial-enriched cell preparations demonstrated a greater ability to incorporate [2-¹⁴C]mevalonic acid into isoprenoid material than did neuronal-enriched preparations. Approximately 4 h were required for maximal uptake of labelled mevalonate by the glial preparations. Further metabolism of the isoprenoid material, involving squalene turnover and sterol demethylation, was still evident even after 15 h of incubation. In vivo, sterol biosynthesis was studied by intraperitoneal injection of sodium [2-¹⁴C]acetate and [U-¹⁴C]glucose, sacrifice of the animals at 2 or 24 h, subsequent isolation of glial- and neuronal-cell enriched fractions and analysis of labelled isoprenoid material. Glial-enriched fractions again contained the bulk of the labelled isoprenoid material.     

Metabolism of acetyl-CoA by isolated peroxisomal fractions: formation of acetate and acetoacetyl-CoA.

Liver peroxisomal fractions, isolated from rats treated with clofibrate, were shown to hydrolyze added [1-14C]acetyl-CoA to free [1-14C]acetate. [1-14C]Acetyl-CoA was, however, also converted to [14C]acetoacetyl-CoA. This reaction was inhibited by added ATP and by solubilization of the peroxisomes. The effect of ATP on synthesis of [14C]acetoacetyl-CoA was likely due to ATP-dependent stimulation of acetyl-CoA hydrolase (EC 3.1.2.1) activity. The inhibitory effect due to solubilizing conditions of incubation remains unexplained. During peroxisomal beta-oxidation of [1-14C]palmitoyl-CoA, [1-14C]acetyl-CoA, [1-14C]acetate, and [14C]acetoacetyl-CoA were shown to be produced. Possible metabolic implications of peroxisomal acetoacetyl-CoA synthesis are discussed.     

In vivo Biosynthesis of isopentenylacetophenones in Eupatorium rugosum

The biosynthesis of dehydrotremetone in Eupatorium rugosum has been investigated by feeding radioactive precursors to intact plants. The carbon atoms of acetate-[1-¹⁴C] and acetate-[2-¹⁴C] were identified in dehydrotremetone by degradation of the molecule. From the pattern of labeling it was concluded that the acetophenone moiety was derived from acetate via the polyacetate pathway. From the incorporation of mevalonate it appeared that the furan ring and its side chain were formed from an isoprenoid compound. Potential aromatic intermediates were chemically synthesized and also fed to plants but only tremetone was found to be efficiently incorporated into dehydrotremetone. Neither 4-hydroxyacetophenone nor 4-hydroxy-3[isopenten-(2)-yl]-acetophenone were efficiently incorporated into dehydrotremetone.     

Isoprenoid synthesis during the cell cycle. Studies of 3-hydroxy-3-methylglutaryl-coenzyme A synthase and reductase and isoprenoid labeling in cells synchronized by centrifugal elutriation

The activities of 3-hydroxy-3-methylglutaryl-coenzyme A synthase and reductase were assayed in exponentially growing LM fibroblasts and Friend murine erythroleukemia cells isolated at various stages of the cell cycle by centrifugal elutriation. The activities of these enzymes were similar in all phases of the cell cycle, regardless of whether the cells were cultured in the presence or absence of serum. These observations were confirmed in murine erythroleukemia cells synchronized by recultivation of pure populations of G1 cells. The incorporation of [14C]acetate or 3H2O into sterols decreased by 30-50% in later stages of the cell cycle, whereas the incorporation of [14C]acetate into ubiquinone increased as the cells progressed toward mitosis. Similar changes in the labeling of sterols compared to ubiquinone and dolichol were observed when [3H]mevalonate was used, suggesting that cell cycle-dependent alterations may occur in the flux of farnesyl pyrophosphate into the various branches of the isoprenoid pathway. Synchronized murine erythroleukemia cells incorporated [3H]mevalonate into protein-bound isoprenyl groups at all stages of the cell cycle, and there were no substantial changes in the electrophoretic profiles of these labeled polypeptides. The finding that the activities of the enzymes regulating mevalonate synthesis did not vary substantially during the cell cycle implies that changes in the endogenous mevalonate pool probably do not play a limiting role in regulating cell cycle traverse when cells are undergoing exponential growth. Although small cell cycle-dependent changes may occur in the relative activity of various post-mevalonate branches of the isoprenoid biosynthetic pathway, there is no evidence that synthesis of any major isoprenoid end product is confined exclusively to a specific phase of the cell cycle.     

Metabolic extraction of dopamine in the canine lungs

Pulmonary metabolism has been demonstrated to be one of the mechanisms of intrapulmonary removal of dopamine. After a bolus injection of [14C]dopamine into the caval vein of an anaesthetized dog a removal of [14C]-radioactivity into the extravascular space was accompanied by an intravascular alteration of the composition of the [14C]compounds during a single transpulmonary passage. A removal of 22% of [14C]-radioactivity out of the bloodstream, together with a metabolic conversion of the remaining [14C]dopamine within the bloodstream, resulted in a total pulmonary extraction of 22% + (0.78 x 18%) = 36%. This report gives a design for further investigation of the metabolic function of the lung sustained by a scheme for problem solving. Also perspectives for clinical application have been compiled.     

The operation of the γ-aminobutyrate bypath of the tricarboxylic acid cycle in brain tissue in vitro

1. Cerebral-cortex slices prelabelled with gamma-amino[1-(14)C]butyrate (GABA) were incubated in a glucose-saline medium. After the initial rapid uptake there was no appreciable re-entry of (14)C into the GABA pool, either from the medium or from labelled metabolites formed in the tissue. The kinetic constants of GABA metabolism were determined by computer simulation of the experimental results by using mathematical procedures. The GABA flux was estimated to be 0.03mumol per min/g, or about 8% of the total flux through the tricarboxylic acid cycle. It was found that the assumption of compartmentation did not greatly affect the estimates of the GABA flux. 2. The time-course of incorporation of (14)C into amino acids associated with the tricarboxylic acid cycle was followed with [1-(14)C]GABA and [U-(14)C]-glucose as labelled substrates. The results were consistent with the utilization of GABA via succinate. This was confirmed by determining the position of (14)C in the carbon skeletons of aspartate and glutamate formed after the oxidation of [1-(14)C]GABA. These results also indicated that under the experimental conditions the reversal of reactions catalysed by alpha-oxoglutarate dehydrogenase and glutamate decarboxylase respectively was negligible. The conversion of [(14)C]GABA into gamma-hydroxybutyrate was probably also of minor importance, but decarboxylation of oxaloacetate did occur at a relatively slow rate. 3. When [1-(14)C]GABA was the labelled substrate there was evidence of a metabolic compartmentation of glutamate since, even before the peak of the incorporation of (14)C into glutamate had been reached, the glutamine/glutamate specific-radioactivity ratio was greater than unity. When [U-(14)C]glucose was oxidized this ratio was less than unity. The heterogeneity of the glutamate pool was indicated also by the relatively high specific radioactivity of GABA, which was comparable with that of aspartate during the whole incubation time (40min). The rates of equilibration of labelled amino acids between slice and medium gave evidence that the permeability properties of the glutamate compartments labelled as a result of oxidation of [1-(14)C]GABA were different from those labelled by the metabolism of [(14)C]glucose. The results showed therefore that in brain tissue incubated under the conditions used, the organization underlying metabolic compartmentation was preserved. The observed concentration ratios of amino acids between tissue and medium were also similar to those obtaining in vivo. These ratios decreased in the order: GABA>acidic acids>neutral amino acids>glutamine. 4. The approximate pool sizes of the amino acids in the different metabolic compartments were calculated. The glutamate content of the pool responsible for most of the labelling of glutamine during oxidation of [1-(14)C]GABA was estimated to be not more than 30% of the total tissue glutamate. The GABA content of the ;transmitter pool' was estimated to be 25-30% of the total GABA in the tissue. The structural correlates of metabolic compartmentation were considered.     

Hormonal regulation of the alpha-ketoglutarate dehydrogenase complex in the isolated perfused rat liver

The metabolic flux through the alpha-ketoglutarate dehydrogenase reaction in perfused livers was monitored by measuring the rate of 14CO2 production from [1-14C]alpha-ketoglutarate. The rates of 14CO2 production and glucose production from [1-14C]alpha-ketoglutarate were increased with increasing perfusate alpha-ketoglutarate concentrations. Vasopressin, angiotensin II, and the alpha 1-adrenergic agonist phenylephrine stimulated transiently by 2.5-fold the metabolic flux through the alpha-ketoglutarate dehydrogenase reaction in the presence and absence of Ca2+ in the perfusion medium. High concentrations of glucagon (1 x 10(-8) M) and 8-p-chlorophenylthio-cAMP (100 microM) (data not shown) also stimulated transiently the metabolic flux through the alpha-ketoglutarate dehydrogenase reaction. However, lower glucagon concentrations (1 x 10(-9) M) stimulated the rate of 14CO2 production from [1-14C]alpha-ketoglutarate only under conditions optimized to fix the cellular oxidation-reduction state at an intermediate level, when glucagon (1 x 10(-9) M)-mediated elevation of cAMP content was greater than that observed under highly oxidizing and reducing conditions. These data indicate that agonists which increase cytosolic free Ca2+ levels stimulate the metabolic flux through the alpha-ketoglutarate dehydrogenase complex. Furthermore, the data presented here demonstrate for the first time that physiological glucagon concentrations stimulate the metabolic flux through the alpha-ketoglutarate dehydrogenase reaction only under conditions known to be optimal for glucagon-mediated Ca2+ mobilization in the isolated perfused rat liver.     

A study on the precursors of the acetyl moiety of acetylcholine in brain slices. Observations on the compartmentalization of the acetyl-coenzyme A pool

1. A method was devised for the determination of the specific radioactivity of the acetyl moiety of acetylcholine synthesized from various (14)C-labelled substrates. 2. The precursor for the acetyl moiety of acetylcholine was studied in slices of striatum and cerebral cortex from rat and guinea-pig brain. Incorporation of radioactivity into acetylcholine was determined after incubating the slices in the presence of [2-(14)C]acetate, [(14)C]bicarbonate, [1,5-(14)C]citrate, dl-[1- or 5-(14)C]glutamate or [1- or 2-(14)C]pyruvate. 3. After incubation for 1h, acetylcholine was accumulated significantly in both striatum slices (4.1nmol/mg of protein) and cerebral-cortex slices (0.57nmol/mg of protein) from the rat. Final concentrations were about 11 and 5 times respectively the initial values. 4. With slices from rat striatum, rat cerebral cortex and guinea-pig cerebral cortex, the specific radioactivity of acetylcholine derived from [2-(14)C]pyruvate was very high, reaching approx. 30, 20 and 6% respectively of the initial specific radioactivity of added pyruvate in the medium. With the striatum slices this high value was reached after incubation for 15min. Incorporation of radioactivity from [2-(14)C]acetate was only 1.25, 5.3 and 19.7% of that from [2-(14)C]pyruvate in rat striatum, rat cerebral-cortex and guinea-pig cerebral-cortex slices respectively. A small but definite incorporation was found from [5-(14)C]glutamate. No incorporation was found from the other substrates. The findings suggest that pyruvate is the most important precursor for the synthesis of the acetyl moiety of acetylcholine in brain slices. 5. The specific radioactivity of acetylcholine relative to that of citrate when [2-(14)C]pyruvate was used compared with that obtained when [2-(14)C]acetate was used. A marked difference was found in all slices, suggesting metabolic compartmentation of the acetyl-CoA pool.     

Oxidation of fatty acids by Leishmania braziliensis panamensis

Heating cultures of Leishmania braziliensis panamensis (grown at 26 degrees C) to 34 degrees C for 1.5-12 h transformed the cells to an ellipsoidally shaped form. The heat treatment caused an increase in the rate of oxidation of both medium and long chain fatty acids but decreased the rate of oxidation of [1-14C]glucose. The rate of fatty acid oxidation continued to increase for times as long as 20 h after returning the cultures to 26 degrees C. In both the promastigote and heat-induced ellipsoidal forms, the ratio of 14CO2 release from [1-14C]laurate to that from [12-14C]laurate was generally larger than four, whereas this ratio from [1-14C]oleate relative to [10-14C]oleate was approximately two. These data show that metabolic and morphological differentiation begin after a short heat treatment and that some metabolic changes may continue even after the reverse transformation is initiated. The data also suggest that either the omega-terminal portion of the fatty acids is not completely oxidized to acetyl CoA and/or that there are two functional fatty acid oxidation pathways in Leishmania.     

Experiments in marine biochemistry. Homarine metabolism in Penaeus duorarum.

A fractionation procedure has been developed which permits the isolation of 1 to 2 mg of homarine from a single shrimp. This procedure was used to show that homarine is endogenously synthesized by Penaeus duorarum in the free unbound form, and to study the metabolic precursors involved. Injected DL-[14C]tryptophan was not converted to [14C]homarine. However, [6-14C]quinolinic acid, a known catabolite of tryptophan, is an effective precursor. [2-14C]Acetate and [U-14C]glycerol are effectively converted to [14C]homarine while [14C]bicarbonate is poorly utilized. The injection of L-[U-14C]aspartate resulted in labeled homarine, but the quantity converted was less than expected. Since [14C]glycerol is an effective precursor there is a possibility that quinolinic acid may be formed in P. duorarum by a condensation similar to that of glyceraldehyde 3-phosphate with aspartic acid or a closely related metabolite. It is suggested that decarboxylation of quinolinic acid gives rise to picolinic acid which is methylated to yield homarine. L-[methyl-14C]Methionine efficiently provides the N-methyl carbon presumably via S-adenosylmethionine.     

Carboxyl-terminal isoprenylation of ras-related GTP-binding proteins encoded by rac1, rac2, and ralA

Membrane localization of p21ras is dependent upon its posttranslational modification by a 15-carbon farnesyl group. The isoprenoid is linked to a cysteine located within a conserved carboxyl-terminal sequence termed the "CAAX" box (where C is cysteine, A is an aliphatic amino acid, and X is any amino acid). We now show that three GTP-binding proteins encoded by the recently identified rac1, rac2, and ralA genes also undergo isoprenoid modification. cDNAs coding for each protein were transcribed in vitro, and the RNAs were translated in reticulocyte lysates. Incorporation of isoprenoid precursors, [3H]mevalonate or [3H]farnesyl pyrophosphate, indicated that the translation products were modified by isoprenyl groups. A protein recognized by an antibody to rac1 also comigrated with a protein metabolically labeled by a product of [3H] mevalonate in cultured cells. Gel permeation chromatography of radiolabeled hydrocarbons released from the rac1, rac2, and ralA proteins by reaction with Raney nickel catalyst indicated that unlike p21Hras, which was modified by a 15-carbon moiety, the rac and ralA translation products were modified by 20-carbon isoprenyl groups. Site-directed mutagenesis established that the isoprenylated cysteines in the rac1, rac2, and ralA proteins were located in the fourth position from the carboxyl terminus. The three-amino acid extension distal to the cysteine was required for this modification. The isoprenylation of rac1 (CSLL), ralA (CCIL), and the site-directed mutants rac1 (CRLL) and ralA (CSIL), demonstrates that the amino acid adjacent to the cysteine need not be aliphatic. Therefore, proteins with carboxyl-terminal CXXX sequences that depart from the CAAX motif should be considered as potential targets for isoprenoid modification.     

Increased heat production proportional to oxygen consumption in human neutrophils activated with phorbol-12-myristate-13-acetate

The heat produced by neutrophils was measured with a flow microcalorimeter. 02 consumption, ATP concentration, lactic acid production and 14CO2 production from oxidation of [1-(14)C]-glucose [6-(14)C]-glucose and [U-14C]-glucose were evaluated. Experiments were also carried out in the presence of the metabolic inhibitors, N-ethylmaleimide and NaF. Heat effects were correlated to the enthalpy change of aerobic and anaerobic glucose catabolism. Two different heat contributions related to two different nonmitochondrial 02 reduction pathways are present during the metabolic burst. Theoretical and experimental data indicate that the reducing power is derived from the catabolism of glucose both through the hexose monophosphate shunt and glycolysis.     

Regulation of gluconeogenesis from pyruvate and lactate in the isolated perfused rat liver

The effects of glucagon and the alpha-adrenergic agonist, phenylephrine, on the rate of 14CO2 production and gluconeogenesis from [1-14C]lactate and [1-14C]pyruvate were investigated in isolated perfused livers of 24-h-fasted rats. Both glucagon and phenylephrine stimulated the rate of 14CO2 production from [1-14C]lactate but not from [1-14C]pyruvate. Neither glucagon nor phenylephrine affected the activation state of the pyruvate dehydrogenase complex in perfused livers derived from 24-h-fasted rats. 3-Mercaptopicolinate, an inhibitor of the phosphoenolpyruvate carboxykinase reaction, inhibited the rates of 14CO2 production and glucose production from [1-14C]lactate by 50% and 100%, respectively. Furthermore, 3-mercaptopicolinate blocked the glucagon- and phenylephrine-stimulated 14CO2 production from [1-14C]lactate. Additionally, measurements of the specific radioactivity of glucose synthesized from [1-14C]lactate, [1-14C]pyruvate and [2-14C]pyruvate indicated that the 14C-labeled carboxyl groups of oxaloacetate synthesized from 1-14C-labeled precursors were completely randomized and pyruvate----oxaloacetate----pyruvate substrate cycle activity was minimal. The present study also demonstrates that glucagon and phenylephrine stimulation of the rate of 14CO2 production from [1-14C]lactate is a result of increased metabolic flux through the phosphoenolpyruvate carboxykinase reaction, and phenylephrine-stimulated gluconeogenesis from pyruvate is regulated at step(s) between phosphoenolpyruvate and glucose.