Previous work from our laboratory (Zinser, E., Paltauf, F., and Daum, G. (1993) J. Bacteriol. 175, 2853-2858) demonstrated steryl ester hydrolase activity in the plasma membrane of the yeast Saccharomyces cerevisiae. Here, we show that the gene product of YEH2/ YLR020c, which is homologous to several known mammalian steryl ester hydrolases, is the enzyme catalyzing this reaction. Deletion of yeast YEH2 led to complete loss of plasma membrane steryl ester hydrolase activity whereas overexpression of the gene resulted in a significant elevation of the activity. Purification of enzymatically active Yeh2p close to homogeneity unambiguously identified this protein as a steryl ester hydrolase and thus as the first enzyme of this kind characterized in S. cerevisiae. In addition to evidence obtained in vitro experiments in vivo contributed to the characterization of this novel enzyme. Sterol analysis of yeh2⌬ unveiled a slightly elevated level of zymosterol suggesting that the esterified form of this sterol precursor is a preferred substrate of Yeh2p. However, in strains bearing hybrid proteins with strongly enhanced Yeh2p activity decreased levels of all steryl esters were observed. Thus, it appears that Yeh2p activity is not restricted to distinct steryl esters but rather has broad substrate specificity. The fact that in a yeh2⌬ deletion strain bulk steryl ester mobilization occurred at a similar rate as in wild type suggested that Yeh2p is not the only steryl ester hydrolase but that other enzymes with overlapping function exist in the yeast.Most eukaryotic cells contain and synthesize sterols, which are essential lipid components of membranes. Besides their role in maintaining membrane permeability and fluidity, effects of sterols on aerobic metabolism (1, 2), completion of the cell cycle (3) as well as on sterol uptake (4) and sterol transport (5) have been described. Considering the different functions of sterols it is obvious that sterol homeostasis, including sterol biosynthesis, uptake, transport, storage, utilization, and efflux, has to be a strictly regulated process. Specific sterols present in different eukaryotic cells vary, ergosterol being the major sterol in the yeast Saccharomyces cerevisiae.Besides other regulatory mechanisms esterification of sterols and hydrolysis of steryl esters (STE) 1 play an important role in cellular sterol homeostasis. These processes do not only allow cells to store chemical energy, which can be used in times of deprivation, but also provide an additional means to balance the concentrations of free sterols and fatty acids, which are essential and critical for cell structure and function. In mammalian cells esterification of sterols is catalyzed either by lecithin:cholesteryl acyltransferase (reviewed in Refs. 6 -8) or by the two acyl-CoA:cholesteryl acyltransferases ACAT1 and ACAT2 (reviewed in Ref. 9). Cholesteryl ester hydrolysis occurs by a number of mammalian enzymes many of which have already been identified, among them carboxyl ester lipase (CEL), lysosomal acid lipase...

In the yeast Saccharomyces cerevisiae, three enzymes of the sterol biosynthetic pathway, namely Erg1p, Erg6p and Erg7p, are located in lipid particles. Whereas Erg1p (squalene epoxidase) is also present in the endoplasmic reticulum (ER) to a significant amount, only traces of Erg6p (sterol C-24 methyltransferase) and Erg7p (lanosterol synthase) are found in the ER. We have chosen these three Erg-proteins as typical representatives of lipid particle proteins to study targeting to their destination. Lipid particle proteins do not contain obvious targeting motifs, but the only common structural feature is the presence of one or two hydrophobic domains near the C-termini. We constructed truncated versions of Erg1p, Erg6p and Erg7p to test the role of these hydrophobic domains in subcellular distribution. Our results demonstrate that lack of the hydrophobic domains prevents at least in part the association of the proteins with lipid particles and causes their retention to the ER. This result strongly supports the view that ER and lipid particles are related organelles.

Since energy storage is a basic metabolic process, the synthesis of neutral lipids occurs in all kingdoms of life. The yeast, Saccharomyces cerevisiae, widely accepted as a model eukaryotic cell, contains two classes of neutral lipids, namely steryl esters and triacylglycerols. Triacylglycerols are synthesized through two pathways governed by the acyl-CoA diacylglycerol acyltransferase Dga1p and the phospholipid diacylglycerol acyltransferase Lro1p, respectively. Steryl esters are formed by the two steryl ester synthases Are1p and Are2p, two enzymes with overlapping function which also catalyze triacylglycerol formation, although to a minor extent. Storage of neutral lipids is tightly linked to the biogenesis of so called lipid particles. The role of this compartment in lipid homeostasis and its interplay with other organelles involved in neutral lipid dynamics, especially the endoplasmic reticulum and the plasma membrane, are subject of current investigations. In contrast to neutral lipid formation, mobilization of triacylglycerols and steryl esters in yeast are less characterized at the molecular level. Only recently, the triacylglycerol lipase Tgl3p was identified as the first yeast enzyme of this kind by function. Genes and gene products governing steryl ester mobilization still await identification. Besides biochemical properties of enzymes involved in yeast neutral lipid synthesis and degradation, regulatory aspects of these pathways and cell biological consequences of neutral lipid depletion will be discussed in this minireview.

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Cefodizime was evaluated for its effect on a number of parameters of leukocyte function in humans. Four healthy volunteers received 2 g i.v. b.i.d. for seven days. Leukocyte activity was measured before, during and after treatment. Using opsonized zymosan as a stimulant, no effect on the respiratory burst of granulocytes was observed. It was found, however, that the lymphocytes in three of the four subjects showed significantly more marked proliferation rates in the mixed lymphocyte reaction after administration of cefodizime than at baseline. The stimulation indices subsequently returned to normal. This pilot study therefore demonstrated that cefodizime has biological response modifying properties in healthy humans.