Abstract. Sterol carrier protein-2 (SCP-2) is a nonenzymatic protein of 13.5 kD which has been shown in in vitro experiments to be required for several stages in cholesterol utilization and biosynthesis. The subcellular localization of SCP-2 has not been definitively established. Using afffinity-purified rabbit polyclonal antibodies against electrophoretically pure SCP-2 from rat liver, we demonstrate by immunoelectron microscopic labeling of ultrathin frozen sections of rat liver that the largest concentration of SCP-2 is inside peroxisomes. In addition the immunolabeling indicates that there are significant concentrations of SCP-2 inside mitochondria, and associated with the endoplasmic reticulum and the cytosol, but not inside the Golgi apparatus, lysosomes, or the nucleus. These results were confirmed by immunoblotting experiments with proteins from purified subcellular fractions of the rat liver ceils carried out with the anti-SCP-2 antibodies. The large concentration of SCP-2 inside peroxisomes strongly supports the proposal that peroxisomes are critical sites of cholesterol utilization and biosynthesis. The presence of SCP-2 inside peroxisomes and mitochondria raises questions about the mechanisms involved in the differential targeting of SCP-2 to these organelles.

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The concept of lethal synthesis as suggested by Peters is reviewed in the light of the more recent work in this area. It is suggested that fluorocitrate is a "suicide" substrate for aconitase rather than a competitive inhibitor as originally suggested. The use of these substances to study glial-neuronal relationships is considered.

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Glutamate dehydrogenase (GDH), an enzyme that is central to the metabolism of glutamate, is present at high levels in the mammalian brain. Studies on human leukocytes and rat brain suggested the presence of two GDH activities differing in thermal stability and allosteric regulation, but molecular biological investigations led to the cloning of two human GDH-specific genes encoding highly homologous polypeptides. The first gene, designated GLUD1, is expressed in all tissues (housekeeping GDH), whereas the second gene, designated GLUD2, is expressed specifically in neural and testicular tissues. In this study, we obtained both GDH isoenzymes in pure form by expressing a GLUD1 cDNA and a GLUD2 cDNA in Sf9 cells and studied their properties. The enzymes generated showed comparable catalytic properties when fully activated by 1 mM ADP. However, in the absence of ADP, the nerve tissue-specific GDH showed only 5% of its maximal activity, compared with -~40%showed by the housekeeping enzyme. Low physiological levels of ADP (0.05-0.25 mM) induced a concentration-dependent enhancement of enzyme activity that was proportionally greater for the nerve tissue GDH (by 550-1,300%) than of the housekeeping enzyme (by 120-150%). Magnesium chloride (1-2 mM) inhibited the nonactivated housekeeping GDH (by 45-64%); this inhibition was reversed almost completely by ADP. In contrast, Mg 2~did not affect the nonstimulated nerve tissuespecific GDH, although the cation prevented much of the allosteric activation of the enzyme at low ADP levels (0.05-0.25 mM). Heat-inactivation experiments revealed that the half-life of the housekeeping and nerve tissuespecific GDH was 3.5 and 0.5 h, respectively. Hence, the nerve tissue-specific GDH is relatively thermolabile and has evolved into a highly regulated enzyme. These allosteric properties may be of importance for regulating brain glutamate fluxes in vivo under changing energy demands. Key Words: Glutamate dehydrogenase-Human brain -Glutamate metabolism-ADP enzyme regulation-Enzyme thermolability.