Cytochrome reductase activities in rat brain microsomes during development

Expression analysis of the mixed function oxidase system in rat brain by the polymerase chain reaction

Metabolism of therapeutic drugs in the body by the mixed function oxidase system is an important consideration in the analysis of a drug's effectiveness. P450-dependent metabolism within the brain of a neuro-specific drug may affect the drug's course of action. To determine whether cytochrome P450 was expressed in brain, RNA was isolated from the whole brains of rats treated with a variety of known hepatic P450 inducers, including amitriptyline, imipramine, isosafrole, phenobarbital, and beta-naphthoflavone. The RNA was analyzed for the presence of P450 isozymes by the PCR technique. Differential expression of P450IA1, P450IIB1, P450IIB2, P450IID, and P450IIE1 was detected in the brain samples, depending on the treatment. Cytochrome P450 reductase expression was also detected in the brain samples, giving strong evidence that the brain contains a competent mixed function oxidase system under all conditions studied.

The fatty acid chain elongation system of mammalian endoplasmic reticulum

Much has been learned about FACES of the endoplasmic reticulum since its discovery in the early 1960s. FACES consists of four component reactions, requires the fatty acid to be activated in the form of a CoA derivative, utilizes reducing equivalents in the form of NADH or NADPH, is induced by a fat-free diet, resides on the cytoplasmic surface of the endoplasmic reticulum, appears to function in concert with the desaturase system and appears to exist in multiple forms (either multiple condensing enzymes connected to a single pathway or multiple pathways). FACES has been found in all tissues investigated, namely, liver, brain, kidney, lung, adrenals, retina, testis, small intestine, blood cells (lymphocytes and neutrophils) and fibroblasts, with one exception--the heart has no measurable activity. Yet, much more needs to be learned. The critical, inducible and rate-limiting condensing enzyme has resisted solubilization and purification; the purification of the other components has met with limited success. We know nothing about the site of synthesis of each component of FACES. How is each component enzyme integrated into the endoplasmic reticulum membrane? Is there a single mRNA directing synthesis of all four components or are there four separate mRNAs? How are elongation and desaturation coordinated? What is (are) the physiological regulator(s) of FACES--ADP, AMP, IP3, G-proteins, phosphorylation, CoA, Ca2+, cAMP, none of these? The molecular biology of FACES is only in the fetal stage of development. We are only scratching the surface--it is an undiscovered country.

Drug metabolizing enzymes in the brain and cerebral microvessels

Several families of brain parenchyma and microvessel endothelial cell enzymes can metabolize substrates of exogenous origin. This xenobiotic metabolism includes functionalization and conjugation reactions and results in detoxication, but also possibly in the formation of pharmacologically active or neurotoxic products. The brain is partially protected from chemical insults by the physical barrier formed by the cerebral microvasculature of endothelial cells, which prevents the influx of hydrophilic molecules. These cells provide also, as a result of their drug-metabolizing enzyme activities, a metabolic barrier against penetrating lipophilic substances. The involvement of these enzymatic activities in neurotoxic events, probably responsible for neuronal dysfunctioning and/or death, neurodegenerative diseases and normal aging, is discussed.

Evidence for two separate beta-ketoacyl CoA reductase components of the hepatic microsomal fatty acid chain elongation system in the rat

The hepatic microsomal fatty acid chain elongation system can utilize either NADPH or NADH. Elongation activity, measured as the rate of malonyl CoA incorporation into palmitoyl CoA, was enhanced by a fat-free diet and by bovine serum albumin (BSA) when either cofactor was employed. When the intermediate products were determined, it was observed that in the presence of BSA and NADPH, the predominant product was the saturated elongated fatty acid, whereas in the presence of BSA and NADH, the major intermediate was the beta-ketoacyl derivative. Employing beta-ketostearoyl CoA as substrate, BSA markedly inhibited NADH-supported beta-ketoacyl CoA reductase activity and stimulated NADPH-supported activity. Furthermore, the sum of the NADH-dependent and NADPH-dependent beta-ketoreductase activities approximated the activity obtained when both cofactors were present in the incubation medium, suggesting the existence of two beta-ketoacyl CoA reductases, one using NADH and the other NADPH.

NADPH:cytochrome P-450(c) reductase: biochemical characterization in rat brain and cultured neurons and evolution of activity during development

NADPH:cytochrome P-450 (c) reductase is a microsomal enzyme which is involved in the cytochrome P-450-dependent biotransformation of many exogenous agents as well as of some endogenous molecules. Using cytochrome c as a substrate, the kinetic parameters of this enzyme were determined in brain microsomes. The comparison of the NADPH:cytochrome P-450 reductase's Vmax values and cytochrome P-450 contents in both fractions, suggests a role of cerebral NADPH:cytochrome P-450 reductase in cytochrome P-450 independent pathways. This is also supported by the different developmental pattern of brain enzyme as compared to the liver enzyme, and by the presence of a relatively high NADPH:cytochrome P-450 reductase activity in immature rat brain and neuronal cultures, while cytochrome P-450 was hardly detectable in these preparations. The enzyme activity was not induced by a phenobarbital chronic treatment neither in the adult brain nor in cultured neurons, suggesting a different regulation of the brain enzyme expression.

UDP galactose:ceramide galactosyltransferase, CDP choline:1,2-diacyl-sn-glycerol phosphocholine transferase, and microsomal reductases in major regions of the developing rat brain in nutritional stress

The influence of nutritional inadequacy during the active growth phase of the developing brain, coinciding with myelinogenesis, was examined in rat pups. Developmental profiles of enzyme activities involved in biosynthesis of myelin membrane lipids, and those of associated pathways which generate precursor compounds for such biosynthetic reactions, were followed as functions of age in major anatomical regions of the brain. It was observed that while galactosyltransferase and choline phosphotransferase activities were significantly diminished, the microsomal cytochrome reductases, which contribute to the process of fatty acid elongation and desaturation, were also lowered. An attempt at adaptative increase of enzyme activity, in response to the stress, was apparent in the case of glycerol-3-phosphate dehydrogenase, which could possibly explain, in part, the lower susceptibility of phospholipid biosynthesis to impairments induced by nutritional insufficiency.

Palmitoyl-CoA elongation in brain microsomes: dependence on cytochrome b5 and NADH-cytochrome b5 reductase

Experiments were performed to demonstrate the involvement of electron transport system in fatty acid elongation in rat brain microsomes. Mercuric chloride and p-chloromercuriphenylsulfonate, inhibitors on NADH-cytochrome b5 reductase, at 32 microM inhibited NADH-supported palmitoyl-CoA elongation to 30 and 60% of control activity, respectively, whereas NADPH-supported palmitoyl-CoA elongation was unaffected by these mercurials. An antibody to rat liver NADH-cytochrome b5 reductase inhibited brain microsomal NADH-cytochrome b5 reductase activity and NADH-dependent palmitoyl-CoA elongation. Treatment of brain microsomes with trypsin diminished the cytochrome b5 content; NADH- and NADPH-cytochrome c reductase activities were significantly decreased, but the decrease in NADH-cytochrome b5 reductase activity was relatively small. Whereas essentially no incorporation of malonyl-CoA into palmitoyl-CoA was observed with trypsin-treated microsomes, addition of detergent-solubilized cytochrome b5 resulted in a recovery of fatty acid elongation. These results indicate the presence of an electron transport system, NADH-NADH-cytochrome b5 reductase-cytochrome b5-fatty acid elongation, in brain microsomes.

Characteristics of b-type cytochromes in brain microsomes: comparison with liver microsomes

Biochemical aspects of b-type cytochromes in swine cerebral microsomes were different from those of cytochrome b5 in liver microsomes, as well as the difference in absorption spectra. First, the kinetic constants, Km and Vmax, in rotenone-insensitive NADH-cytochrome c reductase activity were different from those of liver microsomes, and the activity of cerebral microsomes was higher than that of liver microsomes. Second, midpoint potentials (Em) of b-type cytochromes in cerebral microsomes were measured and compared with liver microsomal cytochrome b5. In cerebral microsomes two components of b-type cytochromes were resolved, and showed Em's of -30 and +50 mV, respectively, in the presence of 2 mM KCN. On the other hand, the Em of liver microsomal cytochrome b5 was -6 mV. The high-potential component of cerebral microsomal b-type cytochromes was identified as brain-b'5 [S. Yoshida, T. Yubisui, and M. Takeshita (1983) Biochem. Int. 7, 291-298] and the low-potential component as brain-b5. The significance of the difference between cerebral and liver microsomal b-type cytochromes was discussed.

Microsomal electron-transport reductase activities and fatty acid elongation in rat brain. Developmental changes, regional distribution and comparison with liver activity

Gestational and postnatal changes of microsomal NADH:cytochrome b5 reductase and NADPH:cytochrome c reductase activities were examined in rat brain. The specific activity of NADH:cytochrome b5 reductase was high at 18-19 days of gestational age, decreased to a minimum at 4 to 6 days after birth and increased thereafter. An essentially similar developmental pattern was observed for the specific activity of NADPH:cytochrome c reductase. In contrast, the specific activities of these reductases in liver microsomes were low, did not display a peak during gestation and increased steadily to a maximum at 40-50 days after birth. The rate of incorporation of [2-14C]malonyl-CoA into palmitoyl-CoA in brain microsomes was found to be high in the foetus, sharply decreased to a minimum at the time of birth and increased thereafter. The activity of fatty acid elongation in liver microsomes was much less than that in brain during gestation and increased rapidly after birth to values at 50-60 days 20-fold greater than the foetal activity. NADH and NADPH were equally effective for brain microsomal fatty acid elongation. Regional distribution of cytochrome reductase activities and the activity of fatty acid elongation showed the lowest specific activity in cerebellum. These results suggest that brain microsomal electron transport may be correlated with the developmental alteration in fatty acid elongation.