Characterization of isoprenoid involved in the post-translational modification of mammalian cell proteins

Dolichol: A Component of the Cellular Antioxidant Machinery

Dolichol, an end product of the mevalonate pathway, has been proposed as a biomarker of aging, but its biological role, not to mention its catabolism, has not been fully understood. UV-B radiation was used to induce oxidative stress in isolated rat hepatocytes by the collagenase method. Effects on dolichol, phospholipid-bound polyunsaturated fatty acids (PL-PUFA) and known lipid soluble antioxidants [coenzyme Q (CoQ) and α-tocopherol] were studied. The increase in oxidative stress was detected by a probe sensitive to reactive oxygen species (ROS). Peroxidation of lipids was assessed by measuring the release of thiobarbituric acid reactive substances (TBARS). Dolichol, CoQ, and α-tocopherol were assessed by high-pressure liquid chromatography (HPLC), PL-PUFA by gas-liquid chromatography (GC). UV-B radiation caused an immediate increase in ROS as well as lipid peroxidation and a simultaneous decrease in the levels of dolichol and lipid soluble antioxidants. Decrease in dolichol paralleled changes in CoQ levels and was smaller to that in α-tocopherol. The addition of mevinolin, a competitive inhibitor of the enzyme 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoAR), magnified the loss of dolichol and was associated with an increase in TBARS production. Changes in PL-PUFA were minor. These findings highlight that oxidative stress has very early and similar effects on dolichol and lipid soluble antioxidants. Lower levels of dolichol are associated with enhanced peroxidation of lipids, which suggest that dolichol may have a protective role in the antioxidant machinery of cell membranes and perhaps be a key to understanding some adverse effects of statin therapy.

Resveratrol Requires Red Wine Polyphenols for Optimum Antioxidant Activity

OBJECTIVE

There is substantial evidence that a diet rich in fruit and vegetables may reduce the risk of aging and stress oxidative associated diseases. It has been suggested that benefits associated with fruit and red wine consumption could be due to pooled antioxidant microcomponents in diet. The aim of this study was to investigate the antioxidant activities of pure resveratrol (a well known phytoalexin, RSV) and red wine polyphenols (RWP), using UV-B radiated isolated rat hepatocytes as a model of oxidative stress.

METHODS

Rat hepatocytes were isolated by the collagenase method. The cells were loaded with resveratrol and/or polyphenols at different concentrations. The production of thiobarbituric acid reactive substances (TBARS) released by UV-B radiated cells and the levels of lipid-soluble antioxidants (Dolichol, Vitamin E, Coenzyme Q9 and Q10) were measured.

RESULTS

Resveratrol had pro-oxidant or antioxidant effects depending on (lower or higher) dosage. RWP protection from photolipoperoxidation was dose-dependent and increased with dosage. Combination of the two compounds exhibited synergistic antioxidant effect, and made resveratrol effective both at lower and higher dosages.

CONCLUSIONS

These results suggest that resveratrol requires red wine polyphenols for optimum antioxidant activity.

Virtual lead identification of farnesyltransferase inhibitors based on ligand and structure-based pharmacophore techniques

Farnesyltransferase enzyme (FTase) is considered an essential enzyme in the Ras signaling pathway associated with cancer. Thus, designing inhibitors for this enzyme might lead to the discovery of compounds with effective anticancer activity. In an attempt to obtain effective FTase inhibitors, pharmacophore hypotheses were generated using structure-based and ligand-based approaches built in Discovery Studio v3.1. Knowing the presence of the zinc feature is essential for inhibitor’s binding to the active site of FTase enzyme; further customization was applied to include this feature in the generated pharmacophore hypotheses. These pharmacophore hypotheses were thoroughly validated using various procedures such as ROC analysis and ligand pharmacophore mapping. The validated pharmacophore hypotheses were used to screen 3D databases to identify possible hits. Those which were both high ranked and showed sufficient ability to bind the zinc feature in active site, were further refined by applying drug-like criteria such as Lipiniski’s “rule of five” and ADMET filters. Finally, the two candidate compounds (ZINC39323901 and ZINC01034774) were allowed to dock using CDOCKER and GOLD in the active site of FTase enzyme to optimize hit selection.

The analytical determination of isoprenoid intermediates from the mevalonate pathway

In this article, assays on the analytical determination of farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP), two important isoprenoid intermediates at biochemically relevant branching points in the mevalonate pathway, are summarized and reviewed. There is considerable recent interest in the measurement of these two isoprenoids because of their direct involvement in several diseases, for example, statins lower cholesterol by inhibiting 3-hydroxy-3-methylglutaryl-CoA reductase but equally affect other metabolite biosyntheses. The isoprenoids FPP and GGPP are key intermediates due to their role as CaaX-specific substrates for posttranslational modification of proteins (protein prenylation). Disease pathologies and therapeutic efficacy of different treatments (e.g., cholesterol-lowering drugs) may lead to a reduction in isoprenoid levels and an accompanying reduction in prenylation of specific proteins. To understand the exact biochemical role of the isoprenoids FPP and GGPP, we need to know their levels. Several recent studies have shown exact levels of FPP and GGP in plasma and relevant tissues and their modulation following treatment. Furthermore, by directly measuring the extent of protein prenylation and identifying target proteins, further insight into the exact biochemical nature of the pathology and regulatory mechanisms will be possible. This short review aims to highlight the relevant literature on the analytical determination of the free isoprenoids FPP and GGPP in biological tissue as well as techniques for directly measuring prenylated proteins.

Effects of oxidative stress on the Dolichol content of isolated rat liver cells

Dolichol, a long-chain polyisoprenoid broadly distributed in all tissues and cellular membranes with unknown function(s), might have a role in free radical metabolism [it accumulates in older tissues and decreases after CCl4 (in liver) or phenylhydrazine (in spleen and liver) administration]. The effects of the NADPH-ADP-Fe system on Dolichol levels in isolated hepatocytes were explored and the time-course of changes was compared with the release of MDA in the incubation medium and the decrease in CoQ 9 and 10 and Vitamin E levels. Results showed that the system increased lipid peroxidation and decreased Dolichol and CoQ levels in-parallel fashions and lowered Vitamin E levels with shorter latency. Meanwhile, no increase in dead cells and no Dolichol release in the medium were detected. In conclusion, an increase in oxidative stress possibly caused a rapid degradation of dolichol by the same (unknown) mechanism responsible for the breakdown of Ubiquinone isoprenoid chains.

Obtaining high sequence coverage in matrix-assisted laser desorption time-of-flight mass spectrometry for studies of protein modification: analysis of human serum albumin as a model

Several approaches were explored for obtaining high sequence coverage in protein modification studies performed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Human serum albumin (HSA, 66.5kDa) was used as a model protein for this work. Experimental factors considered in this study included the type of matrix used for MALDI-TOF MS, the protein digestion method, and the use of fractionation for peptide digests prior to MALDI-TOF MS analysis. A mixture of alpha-cyano-4-hydroxycinnamic acid and 2,5-dihydroxybenzoic acid was employed as the final matrix for HSA. When used with a tryptic digest, this gave unique information on only half of the peptides in the primary structure of HSA. However, the combined use of three enzyme digests based on trypsin, endoproteinase Lys-C, and endoproteinase Glu-C increased this sequence coverage to 72.8%. The use of a ZipTip column to fractionate peptides in these digests prior to analysis increased the sequence coverage to 97.4%. These conditions made it possible to examine unique peptides from nearly all of the structure of HSA and to identify specific modifications to this protein (e.g., glycation sites). For instance, Lys199 was confirmed as a glycation site on normal HSA, whereas Lys536 and Lys389 were identified as additional modification sites on minimally glycated HSA.

Changes in dolichol and pentosidine levels in the age-mismatched heterotopically transplanted rat heart

To address some basic questions about primary and secondary events in the process of aging in different cell and tissue types, we studied changes in the levels of biomarkers of the aging cells (dolichol) and connective tissue (pentosidine) in the heart of older (22-month-old) Lewis rats heterotopically transplanted in younger (3-month-old) syngenic recipients. Results showed that age-mismatched transplantation did not alter the age-related accumulation of dolichol and significantly reduced the accumulation of pentosidine in cardiac tissue. It is concluded that aging of heart muscle and connective tissues is controlled by two independent clocks; that accumulation of dolichol in older tissues may be a primary consequence of the process of aging, whereas the accumulation of pentosidine may be secondary, perhaps to changes in circulating cells endowed with advanced glycation end products-specific receptors; in the perspective of organ transplantation, the environment of a younger host may positively interact with the graft and rejuvenate its collagen.

Proteins are polyisoprenylated in Arabidopsis thaliana

Isoprenoid lipids were found to be covalently linked to proteins of Arabidopsis thaliana. Their identity (polyprenols: Prenol-9-11 with Pren-10 dominating and dolichols: Dol-15-17 with Dol-16 dominating) was confirmed by means of HPLC/ESI-MS with application of the multiple reaction monitoring technique as well as metabolic labeling of Arabidopsis plants with [(3)H]mevalonate and other precursors. The occurrence of typical farnesol-, geranylgeraniol-, and phytol-modified proteins was also noted. Radioisotopic labeling allowed detection of several proteins that were covalently bound to mevalonate-derived isoprenoid alcohols. A significant portion of polyisoprenylated proteins was recovered in the cytosolic/light vesicular fraction of Arabidopsis cells upon subfractionation. Taken together our data prove that a subset of plant proteins is polyisoprenylated.

Dolichol levels in younger and older rat hearts heterotopically transplanted in younger recipients

Dolichol (D) levels increase dramatically in older tissue. An understanding of the exchangeability of D between tissues may be essential in order to understand the mechanism of the abnormal accumulation associated with aging. The question was investigated by the use of organ transplantation. D-poor hearts donated by 3-mon-old and D-rich by 22-mon-old male Lewis rats were transplanted heterotopically in 3-mon-old syngenic recipients, whose peripheral tissues and liver were poor in D. Native and transplanted hearts were taken 7 and 21 d after surgery. Native hearts of 3-mon- and 22-mon-old male Lewis rats served as control. D concentration and quantity were higher in older than in younger native hearts as expected. In the transplanted hearts, the quantity of D was unchanged, irrespective of the age of the donor and of the time of transplantation, whereas D concentration increased because of the remarkable disuse atrophy. No changes in D were observed in recipients' tissues. It is concluded that dolichol is not redistributed via circulation from the transplanted heart to the tissues and liver of the younger recipient.

Effects of ageing and increased haemolysis on the levels of dolichol in rat spleen

Dolichol is a long-chain polyisoprenoid. No enzyme pathway for dolichol degradation was discovered. Dolichol accumulates in human and rodent tissues during ageing. Red blood cells contain a larger amount of dolichol and red blood cell life span is shorter in older rats. The effects of age and of the load of dolichol from red blood cell degradation on the ageing-associated accumulation of dolichol in spleen were studied in 2, 6, 12, 18 and 24 month-old male Sprague Dawley rats fed ad libitum (AL) or on an anti-ageing dietary regimen (EOD). Tissue dolichol was extracted and assayed by HPLC [J. Gerontol. 53A (1998) B87]. Levels of dolichol increased in spleen, liver, kidney and muscle in parallel fashion from the age of 2 to 12 months. Unexpectedly, spleen dolichol decreased in older rats whereas liver, kidney and muscle dolichol increased significantly. The effects of haemolysis on spleen dolichol were tested by the administration of phenylhydrazine. Results show that haemolysis does not increase, but rather decreases the levels of dolichol in erythroclastic organs. It is concluded that the levels of spleen dolichol may decrease in the absence of any known enzymatic degradative pathway if the spleen and its resident phagocytes are forced to cope with a higher number of red blood cells to be cleared. Free-radical mediated decomposition of dolichol by phagocytic cells during erythrophagocytosis might be involved in the process.

Characterization of hydrophobic prenyl groups of isoprenylated proteins in human cancer cells

Extensive protease digestion of delipidated [3H]mevalonate (MVA)-labeled proteins, followed by HPLC separation of the products, is one approach to identify and study prenyl cysteines. Using this methodology three major [3H]MVA-labeled peaks appeared. Two of them represent farnesyl cysteine (FC) and geranylgeranyl cysteine (GGC). The third peak represents unknown products that are considerably more hydrophobic than FC and GGC, here designated HPC. Previously, we provided evidence that cysteine residues may also be modified by dolichyl groups. Dolichyl cysteines (DolC) belong to HPC. However, as shown in the present study, DolC only represents a minor portion of HPC. Data obtained from different sets of experiments, including [3H]GGOH-labeling and use of prenyl transferase inhibitors, suggest that HPC mainly involves CXC or CC residues with double-linked GG groups. In turn this points to the possibility that proteins modified by double GG groups are quite common, and may probably involve other proteins than the rab family of GTPases.

RhoB prenylation is driven by the three carboxyl-terminal amino acids of the protein: evidenced in vivo by an anti-farnesyl cysteine antibody

Protein isoprenylation is a lipid posttranslational modification required for the function of many proteins that share a carboxyl-terminal CAAX motif. The X residue determines which isoprenoid will be added to the cysteine. When X is a methionine or serine, the farnesyl-transferase transfers a farnesyl, and when X is a leucine or isoleucine, the geranygeranyl-transferase I, a geranylgeranyl group. But despite its CKVL motif, RhoB was reported to be both geranylgeranylated and farnesylated. Thus, the determinants of RhoB prenylation appear more complex than initially thought. To determine the role of RhoB CAAX motif, we designed RhoB mutants with modified CAAX sequence expressed in baculovirus-infected insect cells. We demonstrated that RhoB was prenylated as a function of the three terminal amino acids, i.e., RhoB bearing the CAIM motif of lamin B or CLLL motif of Rap1A was farnesylated or geranylgeranylated, respectively. Next, we produced a specific polyclonal antibody against farnesyl cysteine methyl ester allowing prenylation analysis avoiding the metabolic labeling restrictions. We confirmed that the unique modification of the RhoB CAAX box was sufficient to direct the RhoB distinct prenylation in mammalian cells and, inversely, that a RhoA-CKVL chimera could be alternatively prenylated. Moreover, the immunoprecipitation of endogenous RhoB from cells with the anti-farnesyl cysteine antibody suggested that wild-type RhoB is farnesylated in vivo. Taken together, our results demonstrated that the three last carboxyl amino acids are the main determinants for RhoB prenylation and described an anti-farnesyl cysteine antibody as a useful tool for understanding the cellular control of protein farnesylation.

Evidence for protein dolichylation

Labeling of human colon carcinoma cells with [3H]dol, followed by extensive delipidation and removal of dol-P oligosaccharides, showed that dol are bound to cellular proteins with sizes of 5, 10, 27, 75 and > 140 kDa. HPLC purification of proteolytic products of [3H]dol- and [35S]cys-labeled proteins revealed a hydrophobic peak containing both dol and cysteine. The dol/cys-labeled products were clearly separated from GG-cys, and exhibited a hydrophobicity between that of dol-P and dol. In another set of experiments delipidated proteins were treated with methyl iodide, which cleaves thioether bonds. After HPLC purification of released dol-like lipids, these were subjected to mass spectrometry. This demonstrated molecular ions with the same mass as that of dol. Taken together our data provide evidence for the existence of proteins covalently modified with dol.

Protein prenylation in spinach--tissue specificity and greening-induced changes

Etiolated spinach seedlings, as well as petioles and blades of leaves of green seedlings, were labeled with [3H]mevalonate to study protein prenylation in several plant developmental stages. The polypeptide prenylation pattern of the leaf petiole and the leaf blade differed considerably, although some prenylated proteins were present in both tissues. During greening several prenylated polypeptides in the 30- to 46-kDa molecular mass region and two at 15 kDa became more abundant, while others in the 21.5- to 30-kDa region and one at 62 kDa showed a relative decrease. However, the relative amount of several of the prenylated polypeptides did not appear to be altered during the greening process. In etiolated seedlings, more thioether-linked farnesol than geranylgeraniol was found, whereas in seedlings grown under normal light conditions the converse situation prevailed.

Photoreactive analogues of prenyl diphosphates as inhibitors and probes of human protein farnesyltransferase and geranylgeranyltransferase type I

Photoreactive analogues of prenyl diphosphates have been useful in studying prenyltransferases. The effectiveness of analogues with different chain lengths as probes of recombinant human protein prenyltransferases is established here. A putative geranylgeranyl diphosphate analogue, 2-diazo-3,3,3-trifluoropropionyloxy-farnesyl diphosphate (DATFP-FPP), was the best inhibitor of both protein farnesyltransferase (PFT) and protein geranylgeranyltransferase-I (PFFT-I). Shorter photoreactive isprenyl diphosphate analogues with geranyl and dimethylallyl moieties and the DATFP-derivative of farnesyl monophosphate were much poorer inhibitors. DATFP-FPP was a competitive inhibitor of both PFT and PGGT-I with Ki values of 100 and 18 nM, respectively. [32P]DATFP-FPP specifically photoradiolabelled the beta-subunits of both PFT and PGGT-I. Photoradiolabelling of PGGT-I was inhibited more effectively by geranylgeranyl diphosphate than farnesyl diphosphate, whereas photoradiolabelling of PFT was inhibited better by farnesyl diphosphate than geranylgeranyl diphosphate. These results lead to the conclusions that DATFP-FPP is an effective probe of the prenyl diphosphate binding domains of PFT and PGGT-I. Furthermore, the beta-subunits of protein prenyltransferases must contribute significantly to the recognition and binding of the isoprenoid substrate.

Lipid-mediated a-factor interactions with artificial membranes

Our understanding of the cellular export of a-factor and its interaction with the receptor do not yet allow for a description of the phenomena on a molecular level. Synthesis of a-factor analogs and biophysical studies of the lipopeptides in the presence of artificial membranes provide insights which can be analyzed with respect to the biological potency of the molecules. It is through the study of the interaction of the lipopeptides with membranes at varying levels of complexity that we will be able to develop a molecular description of the biological processes.

Substrate characterization of the Saccharomyces cerevisiae protein farnesyltransferase and type-I protein geranylgeranyltransferase

The in vitro substrate preferences of recombinant S. cerevisiae protein farnesyltransferase and type-I protein geranylgeranyl-transferase were determined. Proteins ending in 16 different CaaX sequences (C = cysteine, a = aliphatic amino acid, X = variable amino acids) were used to determine the protein substrate preferences of these S. cerevisiae prenyltransferases. The identities of the attached prenyl groups were confirmed by iodomethane treatment of prenylated substrates and reverse-phase HPLC. The CaaX preference of each recombinant yeast enzyme was found to be nearly identical to the reported preferences of purified mammalian protein farnesyltransferase and type-I protein geranylgeranyltransferase. S. cerevisiae farnesyltransferase preferentially farnesylated CaaX sequences ending in methionine, cysteine or serine. The farnesyltransferase also attached geranylgeranyl to some CaaX sequences ending in methionine, leucine and cysteine. The S. cerevisiae type-I geranylgeranyltransferase preferentially geranylgeranylated CaaX sequences ending in leucine and to a lesser degree methionine. Yeast extracts were found to contain prenylating activities identical to those observed with the recombinant enzymes. Farnesyltransferase activity in yeast extracts exceeded type-I geranylgeranyltransferase activity by at least 3-fold, resulting in prenylation of leucine-ending CaaX substrates with a mixture of 75% geranylgeranyl and 25% farnesyl. These results suggest that some substrate overlap may occur between the S. cerevisiae protein farnesyltransferase and the type-I protein geranylgeranyltransferase in vivo.

The prenylation of proteins

The prenylated proteins represent a newly discovered class of post-translationally modified proteins. The known prenylated proteins include the oncogene product p21ras and other low molecular weight GTP-binding proteins, the nuclear lamins, and the gamma subunit of the heterotrimeric G proteins. The modification involves the covalent attachment of a 15-carbon (farnesyl) or 20-carbon (geranylgeranyl) isoprenoid moiety in a thioether linkage to carboxyl terminal cysteine. The nature of the attached substituent is dependent on specific sequence information in the carboxyl terminus of the protein. In addition, prenylation entrains other posttranslational modifications forming a reaction pathway. In this article, we review our current understanding of the biochemical reactions involved in prenylation and discuss the possible role of this modification in the control of cellular functions such as protein maturation and cell growth.

Cell-cycle-dependent, differential prenylation of proteins

Isoprenylated proteins related to cell growth have been detected during proliferation. Since cholesterogenesis (isoprenoid synthesis) is mandatory for cell proliferation, the observation of a temporally coordinated protein prenylation during the cell division cycle might constitute obligatory processes in the signalling pathway for initiating DNA replication and/or in maintaining the growing state. We have found such a definitive cell-cycle-phase-dependent pattern of prenylation for various classes of cytosolic and nuclear matrix proteins in synchronized HepG2 cells. Characteristic [3H]mevalonate incorporation began to increase during mid-to-late G1, just after cholesterol synthesis reached its apex, and peaked just prior to or coincident with mid S. Incorporation then declined subsequent to S (during G2) as cells approached mitosis. Prior to the rise in mevalonate incorporation into proteins, during early-to-mid G1, steady-state [14C]acetate incorporation into chromatographically resolved cholesterogenic lipid intermediates displayed a maximum only into cholesterol. However, during the late-G1/S interval, a singular peak of 14C incorporation was found for the farnesyl moiety (farnesol/nerolidol plus farnesyl diphosphate). Except for the farnesyl moiety, none of the other polyisoprenoids detected by our procedures showed any fluctuation in 14C incorporation subsequent to mid G1. These results support the proposal that subsequent to peak cholesterol synthesis in early-to-mid G1, the generation of a cholesterol-pathway-dependent set of post-translationally modified, polyisoprenylated proteins could constitute an obligatory step leading to the duplication of the cellular genome, thereby impelling transit through the cell cycle. The well known high flux through cholesterogenesis in tumors, which manifests an intrinsic lack of sensitivity to feedback inhibition and operates continuously, is consonant with this proposal.

Mass spectrometric signature of S-prenylated cysteine peptides

The fast atom bombardment mass spectra of peptides containing S-prenylated cysteine display signature fragmentations characteristic of this modified amino acid. The fragmentation is independent of the nature of the cysteine carbonyl substituent, easily differentiates prenyl from nonprenyl alkylation, and readily identifies the oligomer count of the prenyl. This screening method, which requires little time, effort, or material (compared with previous analysis methods based on chemical degradation), greatly facilitates the identification of these prenylated proteins.

Cell cycling of astrocytes and their precursors in primary cultures: a mevalonate requirement identified in late G1, but before the G1/S transition, involves polypeptides

The relationship between mevalonate and cell cycling was investigated in developing glial cells. Primary cultures of newborn rat brains were serum-depleted (0.1%, vol/vol) for 48 h on days 4-6 in vitro, then returned to 10% calf serum (time 0). After 48 h, 70-80% of the cells were glial fibrillary acidic protein (GFAP)-negative by indirect immunofluorescence; 79 +/- 7% were GFAP-positive after an additional 3 days. Serum shift-up resulted in 12 h of quiescence, and then by 20 h (S phase) in increased proportions of cells synthesizing DNA (from 15 +/- 6% to 75 +/- 4% by bromodeoxyuridine immunofluorescence at 12 h and 20 h, respectively) and rates of DNA synthesis (42 +/- 6 versus 380 +/- 32 cpm/micrograms of protein/h of [3H]thymidine uptake). Additional mevalonate (25 mM) for 30 min at 10 h reversed the inhibition of DNA synthesis apparent with mevinolin (150 microM), an inhibitor of mevalonate synthesis, present from time 0. Cycloheximide added simultaneously with mevalonate prevented this reversal of inhibition. To cause arrest at G1/S, cultures were exposed to hydroxyurea between 10 and 22 h. By 3 h after hydroxyurea removal, bromodeoxyuridine-labeled nuclei increased from 0% to 75 +/- 9%, and DNA synthesis increased 10-fold. Mevinolin failed to inhibit these increases. Thus, primary astroglial precursors stimulated to progress through the cell cycle express a mevalonate requirement in late G1, but before the G1/S transition. The effect of mevalonate was characterized further as being brief (30 min) and as requiring polypeptides.

Active cholesterol biosynthesis in cultured aortic smooth muscle cells: evolution during the life-span of the culture

Cultured aortic smooth muscle cells from rabbit, in synthetic and contractile state, are considered good models for studying pathological and normal cells, respectively, during the atherosclerotic process. Cholesterogenic activity was compared in cells which were obtained in both states of the same subculture and incubated with labeled sodium acetate. This activity (expressed as the percentage of total cell radioactivity uptake transformed into cholesterol) was very high in synthetic cells and comparable to that of cancer cells. Cholesterol synthesis was lower in contractile cells and similar to that observed in a nonpathological cultured cell. During the cell life-span (studied in two cultures) cholesterogenic activity initially increased and then slowly decreased, in the two phenotypic states. Near the end of the culture life, cholesterol production drastically decreased, but this was due to a blocking of the last steps, lanosterol demethylation and C27 sterol transformation into cholesterol, rather than to a sharp decrease in the first steps of the cholesterogenic process. Cells in the synthetic and contractile states released newly synthesized lipids which were essentially late precursors of cholesterol, but accumulation of oxy-sterols was not observed. The excretion of metabolites increased with culture aging.

Prenyl proteins in eukaryotic cells: a new type of membrane anchor

Recent studies have indicated that eukaryotic cells contain proteins that are post-translationally modified by long-chain, thioether-linked prenyl groups. These proteins include yeast mating factors, ras proteins and nuclear lamins. The modification occurs on a cysteine residue near the C terminus and appears to initiate a set of additional protein modification reactions that promote attachment of the proteins to specific membranes.