Relationship between the endoplasmic reticulum-Golgi membrane system and ubiquinone biosynthesis

Probing the Role of Cysteine Thiyl Radicals in Biology: Eminently Dangerous, Difficult to Scavenge

Thiyl radicals are exceptionally interesting reactive sulfur species (RSS), but rather rarely considered in a biological or medical context. We here review the reactivity of protein thiyl radicals in aqueous and lipid phases and provide an overview of their most relevant reaction partners in biological systems. We deduce that polyunsaturated fatty acids (PUFAs) are their preferred reaction substrates in lipid phases, whereas protein side chains arguably prevail in aqueous phases. In both cellular compartments, a single, dominating thiyl radical-specific antioxidant does not seem to exist. This conclusion is rationalized by the high reaction rate constants of thiyl radicals with several highly concentrated substrates in the cell, precluding effective interception by antioxidants, especially in lipid bilayers. The intractable reactivity of thiyl radicals may account for a series of long-standing, but still startling biochemical observations surrounding the amino acid cysteine: (i) its global underrepresentation on protein surfaces, (ii) its selective avoidance in aerobic lipid bilayers, especially the inner mitochondrial membrane, (iii) the inverse correlation between cysteine usage and longevity in animals, (iv) the mitochondrial synthesis and translational incorporation of cysteine persulfide, and potentially (v) the ex post introduction of selenocysteine into the genetic code.

Mechanisms and Therapeutic Effects of Benzoquinone Ring Analogs in Primary CoQ Deficiencies

Coenzyme Q (CoQ) is a conserved polyprenylated lipid composed of a redox-active benzoquinone ring and a long polyisoprenyl tail that serves as a membrane anchor. CoQ biosynthesis involves multiple steps, including multiple modifications of the precursor ring 4-hydroxybenzoic acid. Mutations in the enzymes involved in CoQ biosynthesis pathway result in primary coenzyme Q deficiencies, mitochondrial disorders whose clinical heterogenicity reflects the multiple biological function of CoQ. Patients with these disorders do not always respond to CoQ supplementation, and CoQ analogs have not been successful as alternative approaches. Progress made in understanding the CoQ biosynthesis pathway and studies of supplementation with 4-hydroxybenzoic acid ring analogs have opened a new area in the field of primary CoQ deficiencies treatment. Here, we will review these studies, focusing on efficacy of the different 4-hydroxybenzoic acid ring analogs, models in which they have been tested, and their mechanisms of action. Understanding how these compounds ameliorate biochemical, molecular, and/or clinical phenotypes of CoQ deficiencies is important to develop the most rational treatment for CoQ deficient patients, depending on their molecular defects.

Animal Models of Coenzyme Q Deficiency: Mechanistic and Translational Learnings

Coenzyme Q (CoQ) is a vital lipophilic molecule that is endogenously synthesized in the mitochondria of each cell. The CoQ biosynthetic pathway is complex and not completely characterized, and it involves at least thirteen catalytic and regulatory proteins. Once it is synthesized, CoQ exerts a wide variety of mitochondrial and extramitochondrial functions thank to its redox capacity and its lipophilicity. Thus, low levels of CoQ cause diseases with heterogeneous clinical symptoms, which are not always understood. The decreased levels of CoQ may be primary caused by defects in the CoQ biosynthetic pathway or secondarily associated with other diseases. In both cases, the pathomechanisms are related to the CoQ functions, although further experimental evidence is required to establish this association. The conventional treatment for CoQ deficiencies is the high doses of oral CoQ₁₀ supplementation, but this therapy is not effective for some specific clinical presentations, especially in those involving the nervous system. To better understand the CoQ biosynthetic pathway, the biological functions linked to CoQ and the pathomechanisms of CoQ deficiencies, and to improve the therapeutic outcomes of this syndrome, a variety of animal models have been generated and characterized in the last decade. In this review, we show all the animal models available, remarking on the most important outcomes that each model has provided. Finally, we also comment some gaps and future research directions related to CoQ metabolism and how the current and novel animal models may help in the development of future research studies.

CoenzymeQ in cellular redox regulation and clinical heart failure

Coenzyme Q (CoQ) is ubiquitously embedded in lipid bilayers of various cellular organelles. As a redox cycler, CoQ shuttles electrons between mitochondrial complexes and extramitochondrial reductases and oxidases. In this way, CoQ is crucial for maintaining the mitochondrial function, ATP synthesis, and redox homeostasis. Cardiomyocytes have a high metabolic rate and rely heavily on mitochondria to provide energy. CoQ levels, in both plasma and the heart, correlate with heart failure in patients, indicating that CoQ is critical for cardiac function. Moreover, CoQ supplementation in clinics showed promising results for treating heart failure. This review provides a comprehensive view of CoQ metabolism and its interaction with redox enzymes and reactive species. We summarize the clinical trials and applications of CoQ in heart failure and discuss the caveats and future directions to improve CoQ therapeutics.

The multiple roles of coenzyme Q in cellular homeostasis and their relevance for the pathogenesis of coenzyme Q deficiency

Coenzyme Q (CoQ) is a redox active lipid that plays a central role in cellular homeostasis. It was discovered more than 60 years ago because of its role as electron transporter in the mitochondrial respiratory chain. Since then it has become evident that CoQ has many other functions, not directly related to bioenergetics. It is a cofactor of several mitochondrial dehydrogenases involved in the metabolism of lipids, amino acids, and nucleotides, and in sulfide detoxification. It is a powerful antioxidant and it is involved in the control of programmed cell death by modulating both apoptosis and ferroptosis. CoQ deficiency is a clinically and genetically heterogeneous group of disorders characterized by the impairment of CoQ biosynthesis. CoQ deficient patients display defects in cellular bioenergetics, but also in the other pathways in which CoQ is involved. In this review we will focus on the functions of CoQ not directly related to the respiratory chain, and on how their impairment is relevant for the pathophysiology of CoQ deficiency. A better understanding of the complex set of events triggered by CoQ deficiency will allow to design novel approaches for the treatment of this condition.

Regulation of coenzyme Q biosynthesis pathway in eukaryotes

Coenzyme Q (CoQ, ubiquinone/ubiquinol) is a ubiquitous and unique molecule that drives electrons in mitochondrial respiratory chain and an obligatory step for multiple metabolic pathways in aerobic metabolism. Alteration of CoQ biosynthesis or its redox stage are causing mitochondrial dysfunctions as hallmark of heterogeneous disorders as mitochondrial/metabolic, cardiovascular, and age-associated diseases. Regulation of CoQ biosynthesis pathway is demonstrated to affect all steps of proteins production of this pathway, posttranslational modifications and protein-protein-lipid interactions inside mitochondria. There is a bi-directional relationship between CoQ and the epigenome in which not only the CoQ status determines the epigenetic regulation of many genes, but CoQ biosynthesis is also a target for epigenetic regulation, which adds another layer of complexity to the many pathways by which CoQ levels are regulated by environmental and developmental signals to fulfill its functions in eukaryotic aerobic metabolism.

Molecular genetics of ubiquinone biosynthesis in animals

Ubiquinone (UQ), also known as coenzyme Q (CoQ), is a redox-active lipid present in all cellular membranes where it functions in a variety of cellular processes. The best known functions of UQ are to act as a mobile electron carrier in the mitochondrial respiratory chain and to serve as a lipid soluble antioxidant in cellular membranes. All eukaryotic cells synthesize their own UQ. Most of the current knowledge on the UQ biosynthetic pathway was obtained by studying Escherichia coli and Saccharomyces cerevisiae UQ-deficient mutants. The orthologues of all the genes known from yeast studies to be involved in UQ biosynthesis have subsequently been found in higher organisms. Animal mutants with different genetic defects in UQ biosynthesis display very different phenotypes, despite the fact that in all these mutants the same biosynthetic pathway is affected. This review summarizes the present knowledge of the eukaryotic biosynthesis of UQ, with focus on the biosynthetic genes identified in animals, including Caenorhabditis elegans, rodents, and humans. Moreover, we review the phenotypes of mutants in these genes and discuss the functional consequences of UQ deficiency in general.

Non-mitochondrial coenzyme Q

The key role of coenzyme Q (ubiquinone or Q) is in mitochondrial and prokaryotic energetics. Less well investigated is the basis for its presence in eukaryotic membrane locations other than mitochondria and in plasma where both antioxidant and potentially more targeted roles are indicated. Included in the latter is that of a lipid-soluble electron transfer intermediate that serves as the transmembrane component of plasma membrane and Golgi apparatus electron transport, which regulates cytosolic NAD(+) /NADH ratios and is involved in vectorial membrane displacements and in the regulation of cell growth. Important protective effects on circulating lipoproteins and in the prevention of coronary artery disease ensue not only from the antioxidant role of CoQ(10) but also from its ability to directly block protein oxidation and superoxide generation of the TM-9 family of membrane proteins known as age-related NADH oxidase or arNOX (ENOX3) and their shed forms that appear after age 30 and some of which associate specifically with low-density lipoprotein particles to catalyze protein oxidation and crosslinking.

Relationships among the biosyntheses of ubiquinone, carotene, sterols, and triacylglycerols in Zygomycetes

The Zygomycetes Phycomyces blakesleeanus and Blakeslea trispora are actual or potential sources of beta-carotene, ergosterol, ubiquinone, edible oil, and other compounds. By feeding [14C]acetyl-CoA, L-[14C]leucine, or R-[14C]mevalonate in the presence of excess unlabeled glucose, we found that ubiquinone (the terpenoid moiety), beta-carotene, and triacylglycerols were made from separate pools of all their common intermediates; the pools for ubiquinone and ergosterol were indistinguishable. Fatty acids were not labeled from mevalonate, showing the absence in these fungi of a shunt pathway that would recycle carbon from mevalonate and its products back to central metabolism. The overproduction of carotene in a Phycomyces mutant and in sexually mated cultures of Blakeslea modified the relative use of labeled and unlabeled carbon sources in the production of carotene, but not of the other compounds. We concluded that carotene, ubiquinone, and triacylglycerols are synthesized in separate subcellular compartments, while sterols and ubiquinone are synthesized in the same compartments or in compartments that exchange precursors. Carotene biosynthesis was regulated specifically and not by flow diversion in a branched pathway.

Ubiquinone synthesis in mitochondrial and microsomal subcellular fractions of Pneumocystis spp.: differential sensitivities to atovaquone

The lung pathogen Pneumocystis spp. is the causative agent of a type of pneumonia that can be fatal in people with defective immune systems, such as AIDS patients. Atovaquone, an analog of ubiquinone (coenzyme Q [CoQ]), inhibits mitochondrial electron transport and is effective in clearing mild to moderate cases of the infection. Purified rat-derived intact Pneumocystis carinii cells synthesize de novo four CoQ homologs, CoQ7, CoQ8, CoQ9, and CoQ10, as demonstrated by the incorporation of radiolabeled precursors of both the benzoquinone ring and the polyprenyl chain. A central step in CoQ biosynthesis is the condensation of p-hydroxybenzoic acid (PHBA) with a long-chain polyprenyl diphosphate molecule. In the present study, CoQ biosynthesis was evaluated by the incorporation of PHBA into completed CoQ molecules using P. carinii cell-free preparations. CoQ synthesis in whole-cell homogenates was not affected by the respiratory inhibitors antimycin A and dicyclohexylcarbodiimide but was diminished by atovaquone. Thus, atovaquone has inhibitory activity on both electron transport and CoQ synthesis in this pathogen. Furthermore, both the mitochondrial and microsomal fractions were shown to synthesize de novo all four P. carinii CoQ homologs. Interestingly, atovaquone inhibited microsomal CoQ synthesis, whereas it had no effect on mitochondrial CoQ synthesis. This is the first pathogenic eukaryotic microorganism in which biosynthesis of CoQ molecules from the initial PHBA:polyprenyl transferase reaction has been unambiguously shown to occur in two distinct compartments of the same cell.

Metabolism and function of coenzyme Q

Coenzyme Q (CoQ) is present in all cells and membranes and in addition to be a member of the mitochondrial respiratory chain it has also several other functions of great importance for the cellular metabolism. This review summarizes the findings available to day concerning CoQ distribution, biosynthesis, regulatory modifications and its participation in cellular metabolism. There are a number of indications that this lipid is not always functioning by its direct presence at the site of action but also using e.g. receptor expression modifications, signal transduction mechanisms and action through its metabolites. The biosynthesis of CoQ is studied in great detail in bacteria and yeast but only to a limited extent in animal tissues and therefore the informations available is restricted. However, it is known that the CoQ is compartmentalized in the cell with multiple sites of biosynthesis, breakdown and regulation which is the basis of functional specialization. Some regulatory mechanisms concerning amount and biosynthesis are established and nuclear transcription factors are partly identified in this process. Using appropriate ligands of nuclear receptors the biosynthetic rate can be increased in experimental system which raises the possibility of drug-induced upregulation of the lipid in deficiency. During aging and pathophysiological conditions the tissue concentration of CoQ is modified which influences cellular functions. In this case the extent of disturbances is dependent on the localization and the modified distribution of the lipid at cellular and membrane levels.

Are cytochrome b gene mutations the only cause of atovaquone resistance in Pneumocystis?

There is evidence that exposure of the opportunistic pathogen Pneumocystis to atovaquone enhances the development of resistance to the drug. Atovaquone is a structural analog of ubiquinone, which binds to the mitochondrial cytochrome bc(1) complex and inhibits electron transport. Like the parasites Plasmodium and Toxoplasma, atovaquone resistance can result from mutations in the cytochrome b gene of Pneumocystis. However, atovaquone resistance cannot be explained by cytochrome b gene mutations in all cases. The discovery that atovaquone also inhibits biosynthesis of ubiquinone in P. carinii may unfold other mechanisms by which drug resistance develops.

Reduction of ubiquinone by lipoamide dehydrogenase. An antioxidant regenerating pathway

Lipoamide dehydrogenase belongs to a family of pyridine nucleotide disulfide oxidoreductases and is ubiquitous in aerobic organisms. This enzyme also reduces ubiquinone (the only endogenously synthesized lipid-soluble antioxidant) to ubiquinol, the form in which it functions as an antioxidant. The reduction of ubiquinone was linear with time and exhibited turnover numbers of 5 and 1.2 min(-1) in the presence and absence of zinc, respectively. The reaction was stimulated by zinc and cadmium but not by the other divalent ions tested. The zinc/cadmium-dependent stimulation of the reaction increased rapidly and linearly up to a concentration of 0.1 mM and was even further increased at 0.5 mM. At pH 6, the activity was three times higher than at physiological pH. Alteration of the NADPH : NADP(+) ratio revealed that the reaction is inhibited by higher concentrations of the oxidized cofactors. FAD reduced ubiquinone in a dose-dependent manner at a considerably lower rate, suggesting that the reduction of ubiquinone by lipoamide dehydrogenase involves the FAD moiety of the enzyme.

Effects of atovaquone and diospyrin-based drugs on ubiquinone biosynthesis in Pneumocystis carinii organisms

The naphthoquinone atovaquone is effective against Plasmodium and Pneumocystis carinii carinii. In Plasmodium, the primary mechanism of drug action is an irreversible binding to the mitochondrial cytochrome bc(1) complex as an analog of ubiquinone. Blockage of the electron transport chain ultimately inhibits de novo pyrimidine biosynthesis since dihydroorotate dehydrogenase, a key enzyme in pyrimidine biosynthesis, is unable to transfer electrons to ubiquinone. In the present study, the effect of atovaquone was examined on Pneumocystis carinii carinii coenzyme Q biosynthesis (rather than electron transport and respiration) by measuring its effect on the incorporation of radiolabeled p-hydroxybenzoate into ubiquinone in vitro. A triphasic dose-response was observed, with inhibition at 10 nM and then stimulation up to 0.2 microM, followed by inhibition at 1 microM. Since other naphthoquinone drugs may also act as analogs of ubiquinone, diospyrin and two of its derivatives were also tested for their effects on ubiquinone biosynthesis in P. carinii carinii. In contrast to atovaquone, these drugs did not inhibit the incorporation of p-hydroxybenzoate into P. carinii carinii ubiquinone.

Synthesis of mevalonate pathway lipids in fibroblasts from Zellweger and X-linked ALD patients

Fibroblasts were cultured to determine the involvement of peroxisomes in cholesterol and dolichol synthesis. For this purpose, the behavior of cells from patients with Zellweger syndrome, with X-linked adrenoleukodystrophy, and from nondiseased control subjects was studied. Cells both after pretreatment with mevinolin and without pretreatment were incubated in a medium containing [3H]-mevalonate. In fibroblasts from patients with peroxisomal defects, the cholesterol content and mevalonate incorporation into cholesterol were decreased by 10-20% in comparison with control cells. Mevinolin pretreatment decreased the incorporation rate of [3H]-mevalonate into cholesterol but increased the labeling of ubiquinone and dolichol both in diseased and control cells. Squalene synthase activity was unchanged, whereas the activity of farnesyl-pyrophosphate synthase was increased in the diseased states. The results show that in patients with peroxisomal deficiency neither the amount nor the rate of synthesis of cholesterol and dolichol is reduced to any greater extent.

Yeast and rat Coq3 and Escherichia coli UbiG polypeptides catalyze both O-methyltransferase steps in coenzyme Q biosynthesis

Ubiquinone (coenzyme Q or Q) is a lipid that functions in the electron transport chain in the inner mitochondrial membrane of eukaryotes and the plasma membrane of prokaryotes. Q-deficient mutants of Saccharomyces cerevisiae harbor defects in one of eight COQ genes (coq1-coq8) and are unable to grow on nonfermentable carbon sources. The biosynthesis of Q involves two separate O-methylation steps. In yeast, the first O-methylation utilizes 3, 4-dihydroxy-5-hexaprenylbenzoic acid as a substrate and is thought to be catalyzed by Coq3p, a 32.7-kDa protein that is 40% identical to the Escherichia coli O-methyltransferase, UbiG. In this study, farnesylated analogs corresponding to the second O-methylation step, demethyl-Q(3) and Q(3), have been chemically synthesized and used to study Q biosynthesis in yeast mitochondria in vitro. Both yeast and rat Coq3p recognize the demethyl-Q(3) precursor as a substrate. In addition, E. coli UbiGp was purified and found to catalyze both O-methylation steps. Futhermore, antibodies to yeast Coq3p were used to determine that the Coq3 polypeptide is peripherally associated with the matrix-side of the inner membrane of yeast mitochondria. The results indicate that one O-methyltransferase catalyzes both steps in Q biosynthesis in eukaryotes and prokaryotes and that Q biosynthesis is carried out within the matrix compartment of yeast mitochondria.

Ubiquinone redox cycle as a cellular antioxidant defense system

Ubiquinone (UQ) reductase responsible for reduction of non-mitochondrial UQ was investigated in rats toward demonstrating an antioxidant role of UQ. In the liver, most of cellular UQ-10 reductase activity was attributable to a novel NADPH-UQ reductase in cytosol. The enzyme was not inhibited by dicumarol and rotenone, and had a Km of 19 microM for NADPH and 307 microM for NADH at the optimum pH 7.4. The enzyme was purified 300-fold to apparent homogeneity from the liver cytosol by an affinity chromatographic method. The purified enzyme reduced UQ-10 in lecithin liposomes, and protected the liposomes from lipid peroxidation. Furthermore, supplementation of rats with UQ-10 was observed to increase the enzyme level in their livers without affecting levels of other antioxidant factors. The observations suggested that cytosolic NADPH-UQ reductase is responsible for cellular UQ redox cycle as an endogenous antioxidant.

Elevation of ubiquinone content by peroxisomal inducers in rat liver during aging

The effect of di(2-ethylhexyl)phthalate (DEHP) on the induction of peroxisomes and the content of ubiquinone in the liver was studied in rats between 25 and 496 days of age. During this period, peroxisomal beta-oxidation of fatty acids was greatly decreased but it could be induced many-fold in all ages. The ubiquinone (UQ) content was increased upon induction 6-fold in the first weeks of life, but the extent of this elevation continuously narrowed and no induction could be observed in the oldest animals, even after prolonged treatment with the plasticizer. In contrast, the treatment decreased the amount of liver cholesterol in all age groups. Treatment with this peroxisomal inducer increased the biosynthesis of UQ while the breakdown rate was found to be unaffected, as the half-life of this lipid was 103 and 106 h in control and treated rats, respectively. These results indicate that treatment with peroxisomal inducers increases the liver UQ content by increasing the rate of biosynthesis and that this effect is not apparent in aged rats.

Phosphorylation of farnesol in rat liver microsomes: properties of farnesol kinase and farnesyl phosphate kinase

As farnesol may serve as a nonsterol endogenous regulator of the mevalonate pathway, the possibility that a kinase specific for its phosphorylation is present in the rat liver was investigated. In the 10,000 g supernatant of rat liver, farnesyl monophosphate was synthesized in the presence of ATP. The Km value for farnesol was 2.3 microM. Various detergents inhibited the activity of the enzyme. The farnesol kinase was present in rough and in smooth I microsomes, but not in smooth II microsomes, mitochondria, peroxisomes, Golgi, or plasma membranes. The enzyme was associated with the inner, luminal surface of the vesicles. Further analyses have demonstrated that an enzymatic mechanism exists which catalyzes the phosphorylation of farnesyl-P to farnesyl-PP. Activity of the farnesyl phosphate kinase resulted in the phosphorylation of the monophosphate by CTP but not by ATP, GTP, or UTP. This enzyme is activated by low concentrations of detergents. Treatment with proteases and chemical probes indicate that this second phosphorylation reaction probably takes place on the outer, cytoplasmic surface of microsomal vesicles. These results demonstrate that rat liver microsomes contain two enzymes for the consecutive phosphorylation of farnesol to farnesyl-PP.

Mevalonate-derived isopentenyl diphosphate is the biosynthetic precursor of ubiquinone prenyl side chain in tobacco BY-2 cells

Study of the incorporation of 13C-labelled glucose or pyruvate into the isoprenoids of tobacco BY-2 cells allowed the biosynthetic origin of isopentenyl diphosphate to be determined. Sterols synthesized in the cytoplasm and the prenyl chain of ubiquinone Q10 located in mitochondria were derived from the same isopentenyl diphosphate pool, synthesized from acetyl-CoA through mevalonate, whereas the prenyl chain of plastoquinone was obtained from the mevalonate-independent glyceraldehyde 3-phosphate/pyruvate route, like all chloroplast isoprenoids from higher plants. These results are in accord with the compartmentation and complete enzymic independence of the biosynthesis of long-chain all-trans polyprenols in mitochondria and chloroplasts.

Characterization of the COQ5 gene from Saccharomyces cerevisiae. Evidence for a C-methyltransferase in ubiquinone biosynthesis

Ubiquinone (coenzyme Q or Q) is a lipophilic metabolite that functions in the electron transport chain in the plasma membrane of prokaryotes and in the inner mitochondrial membrane of eukaryotes. Q-deficient mutants of Saccharomyces cerevisiae fall into eight complementation groups (coq1-coq8). Yeast mutants from the coq5 complementation group lack Q and as a result are respiration-defective and fail to grow on nonfermentable carbon sources. A nuclear gene, designated COQ5 was isolated from a yeast genomic library based on its ability to restore growth of a representative coq5 mutant on media containing glycerol as the sole carbon source. The DNA segment responsible for the complementation contained an open reading frame (GenBankTM accession number Z49210Z49210) with 44% sequence identity over 262 amino acids to UbiE, which is required for a C-methyltransferase step in the Q and menaquinone biosynthetic pathways in Escherichia coli. Both the ubiE and COQ5 coding sequences contain sequence motifs common to a wide variety of S-adenosyl-L-methionine-dependent methyltransferases. A gene fusion expressing a biotinylated form of Coq5p retains function, as assayed by the complementation of the coq5 mutant. This Coq5-biotinylated fusion protein is located in mitochondria. The synthesis of two farnesylated analogs of intermediates in the ubiquinone biosynthetic pathway is reported. These reagents have been used to develop in vitro C-methylation assays with isolated yeast mitochondria. These studies show that Coq5p is required for the C-methyltransferase step that converts 2-methoxy-6-polyprenyl-1, 4-benzoquinone to 2-methoxy-5-methyl-6-polyprenyl-1,4-benzoquinone.

Squalene synthase inhibition alters metabolism of nonsterols in rat liver

We have used the potent squalene synthase inhibitor squalestatin I to investigate the regulation of isoprenoid metabolism in rat liver Fresh-frozen liver pieces from normal rats and rats infused with squalestatin I at 16 micrograms h-1 for 16 h were assayed for farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) by HPLC after dephosphorylation. Levels of FPP and GGPP were 5.4 +/- 1.6 nmol g-1 and 1.6 +/- 0.7 nmol g-1 (n = 13) wet wt., respectively, in control livers and 110 + 41 nmol g-1 and 3.0 +/- 2.2 nmol g-1 (n = 13) in livers from squalestatin I infused rats. In order to determine the relative level of isopentenyl pyrophosphate, liver slices from normal and squalestatin I infused rats were labeled to steady-state with [3H]acetate. Analysis of isoprenoid pyrophosphate intermediates by radio-HPLC after dephosphorylation indicated that squalestatin I brought about a 20-fold increase in the relative level of FPP (confirming direct analysis) and a 5-fold increase in the relative level of IPP. No change in either of these compounds was observed in livers from cholesterol-fed rats. To determine if squalestatin I altered the synthesis of nonsterol products, rats were subjected to long term subcutaneous infusion. After 14 days of infusion of 15 micrograms h-1, the median chain length of hepatic dolichol and dolichyl phosphate increased from C95 to C115 and the levels of these lipids increased approximately 3-fold. In addition, dolichyl phosphate mannose synthase activity in microsomes from squalestatin I treated rats was increased relative to controls when assayed in the absence of dolichyl phosphate. Squalestatin I affected ubiquinone metabolism to a lesser extent: chain lengths shifted from a Q10/Q9 ratio of 0.118 +/- 0.021 in the normal rat to 0.185 +/- 0.016 in the squalestatin I treated animals, and levels rose by approximately 90%. These results suggest that the isoprenoid pyrophosphate intermediates are shared by the cholesterol, dolichol and ubiquinone pathways and further show that the dolichol and ubiquinone pathways are not saturated. Apparently, under normal conditions, the levels of these intermediates are maintained relatively constant by coordinate enzyme regulation, thereby ensuring a constant rate of synthesis of nonsterols.