CoA-dependent methylmalonate-semialdehyde dehydrogenase, a unique member of the aldehyde dehydrogenase superfamily. cDNA cloning, evolutionary relationships, and tissue distribution

Three overlapping cDNA clones encoding methylmalonate-semialdehyde dehydrogenase (MMSDH; 2-methyl-3-oxopropanoate:NAD+ oxidoreductase (CoA-propanoylating); EC 1.2.1.27) have been isolated by screening a rat liver lambda gt 11 library with nondegenerate oligonucleotide probes synthesized according to polymerase chain reaction-amplified portions coding for the N-terminal amino acid sequence of rat liver MMSDH. The three clones cover a total of 1942 base pairs of cDNA, with an open reading frame of 1569 base pairs. The authenticity of the composite cDNA was confirmed by a perfect match of 43 amino acids known from protein sequencing. The composite cDNA predicts a 503 amino acid mature protein with M(r) = 55,330, consistent with previous estimates. Polymerase chain reaction was used to obtain the sequence of the 32 amino acids corresponding to the mitochondrial entry peptide. Northern blot analysis of total RNA from several rat tissues showed a single mRNA band of 3.8 kilobases. Relative mRNA levels were: kidney greater than liver greater than heart greater than muscle greater than brain, which differed somewhat from relative MMSDH protein levels determined by Western blot analysis: liver = kidney greater than heart greater than muscle greater than brain. A 1423-base pair cDNA clone encoding human MMSDH was isolated from a human liver lambda gt 11 library. The human MMSDH cDNA contains an open reading frame of 1293 base pairs that encodes the protein from Leu-74 to the C terminus. Human and rat MMSDH share 89.6 and 97.7% identity in nucleotide and protein sequence, respectively. MMSDH clearly belongs to a superfamily of aldehyde dehydrogenases and is closely related to betaine aldehyde dehydrogenase, 2-hydroxymuconic semialdehyde dehydrogenase, and class 1 and 2 aldehyde dehydrogenases.

Comparison of ethanol sensitivity of rat brain kainate, DL-alpha-amino-3-hydroxy-5-methyl-4-isoxalone proprionic acid and N-methyl-D-aspartate receptors expressed in Xenopus oocytes

The effect of ethanol (EtOH) on kainate (KA), DL-alpha-amino-3-hydroxy-5-methyl-4-isoxalone proprionic acid and N-methyl-D-aspartate (NMDA) receptor-operated channels was examined electrophysiologically in Xenopus laevis oocytes expressing mRNA from rat hippocampus and cerebellum. EtOH (50, 100 mM) inhibited KA-induced currents but did not alter the EC50 for KA (approximately 78 microM). For a series of n-alcohols, potency for inhibition of KA responses was related to chain length. 6,7-dinitroquinoxaline-2,3-dione inhibited maximum KA responses with an IC50 of approximately 1 microM; EtOH (50, 100 mM) did not alter the IC50 for 6,7-dinitroquinoxaline-2,3-dione but did not produce further inhibition of KA-induced currents. Despite the apparent noncompetitive inhibition produced by EtOH on KA receptor-mediated responses, the EtOH inhibition increased as the KA concentration decreased in hippocampal and cerebellar mRNA expressing oocytes. This differential inhibition was not due to the different current amplitudes stimulated by low vs. high KA concentrations. In contrast, oocytes expressing NMDA channels demonstrated a constant percent inhibition by EtOH in the presence of 25 to 200 microM NMDA. Altering the extracellular Ca++ concentration did not affect the ability of EtOH to inhibit NMDA responses. Maximal NMDA-stimulated currents were inhibited by 100 mM EtOH to a lesser extent (31%) in oocytes injected with rat cerebellar mRNA than oocytes expressing rat hippocampal mRNA (47%), suggesting brain regional differences in NMDA channel inhibition by EtOH.(ABSTRACT TRUNCATED AT 250 WORDS)

Branched-chain alpha-ketoacid dehydrogenase kinase. Molecular cloning, expression, and sequence similarity with histidine protein kinases

A cDNA for branched-chain alpha-ketoacid dehydrogenase kinase was cloned from a rat heart cDNA library. The cDNA had an open reading frame encoding a protein of 382 amino acid residues with a calculated molecular weight of 43,280. The clone codes for the branched-chain alpha-ketoacid dehydrogenase kinase based on the following: 1) the deduced amino acid sequence contained the partial sequence of the kinase determined by direct sequencing; 2) expression of the cDNA in Escherichia coli resulted in synthesis of a 43,000-Da protein that was recognized specifically by kinase antibodies; and 3) enzyme activity that phosphorylated and inactivated the branched-chain alpha-ketoacid dehydrogenase complex was found in extracts of E. coli expressing the protein. Northern blot analysis indicated the mRNA for the branched-chain alpha-ketoacid dehydrogenase kinase was more abundant in rat heart than in rat liver, as expected from the relative amounts of kinase activity expressed in these tissues. The deduced sequence of the kinase aligned with a high degree of similarity within subdomains characteristic of procaryotic histidine protein kinases. This first mitochondrial protein kinase to be cloned appears more closely related in sequence to procaryotic histidine protein kinases than to eucaryotic serine/threonine protein kinases.

Effects of clofibric acid on the activity and activity state of the hepatic branched-chain 2-oxo acid dehydrogenase complex

Feeding clofibric acid to rats caused little or no change in total activity of the liver branched-chain 2-oxo acid dehydrogenase complex (BCODC). No change in mass of liver BCODC was detected by immunoblot analysis in response to dietary clofibric acid. No changes in abundance of mRNAs for the BCODC E1 alpha, E1 beta and E2 subunits were detected by Northern-blot analysis. Likewise, dietary clofibric acid had no effect on the activity state of liver BCODC (percentage of enzyme in the dephosphorylated, active, form) of rats fed on a chow diet. However, dietary clofibric acid greatly increased the activity state of liver BCODC of rats fed on a diet deficient in protein. No stable change in liver BCODC kinase activity was found in response to clofibric acid in either chow-fed or low-protein-fed rats. Clofibric acid had a biphasic effect on flux through BCODC in hepatocytes prepared from low-protein-fed rats. Stimulation of BCODC flux at low concentrations was due to clofibric acid inhibition of BCODC kinase, which in turn allowed activation of BCODC by BCODC phosphatase. Inhibition of BCODC flux at high concentrations was due to direct inhibition of BCODC by clofibric acid. The results suggest that the effects of clofibric acid in vivo on branched-chain amino acid metabolism can be explained by the inhibitory effects of this drug on BCODC kinase.

Activation of protein kinase C selectively inhibits the gamma-aminobutyric acidA receptor: role of desensitization

The effects of protein kinase C (PKC) activators on gamma-aminobutyric acidA (GABAA) receptor function were studied by two-electrode voltage-clamp in Xenopus oocytes expressing brain mRNA or subunit cDNAs and in isolated mouse brain cerebellar membrane vesicles (microsacs), using 36Cl- uptake. Both oocytes and microsacs showed transient (desensitizing) and sustained (nondesensitizing) GABAA receptor responses. In oocytes expressing brain mRNA, the PKC activator phorbol myristoyl acetate (PMA), but not the inactive analog phorbol 12-monomyristate, inhibited both transient and sustained GABA-gated chloride currents. The inhibition by PMA was concentration dependent, with an EC50 of approximately 5 nM, and resulted in a decrease in the efficacy, but not the potency, of GABA. Additionally, PMA inhibited GABA-gated chloride currents in oocytes expressing alpha 1 beta 1 gamma 2L subunit cDNAs. The effect of PMA on recombinant receptors was significantly antagonized by PKC inhibitory peptide (PKCI). In the microsac preparation, the PKC activators (-)-7-octylindolactam V and PMA inhibited the sustained phase of 36Cl- flux without altering the transient phase. The action of PMA was blocked by kinase inhibitors and by depletion of Mg-ATP and was mimicked by protein phosphatase inhibitors. These results demonstrate that activation of PKC inhibits GABAA receptor function, and the results from the microsac experiments suggest that PKC-dependent phosphorylation preferentially inactivates a nondesensitized form or state of the receptor.

Neurobiology of alcohol abuse

Excessive consumption of beverage alcohol (ethanol) is a major health concern worldwide. Understanding the mechanisms by which ethanol affects neural functioning, after both acute and chronic exposure, has become a major goal in the study of alcoholism. With such an understanding, we should be able to institute more effective treatments and preventative measures for alcohol abuse problems. Recent studies have found, contrary to earlier assumptions, that ethanol has selective, dose-dependent effects on various neurotransmitter systems within the CNS. These effects are observed at all levels of analysis, from molecular to behavioral. This review by Herman Samson and Adron Harris covers these recent findings, with the intent of generating questions that will focus further research efforts.

Metabolic effects of proglycosyn

Proglycosyn, a phenylacyl imidazolium compound that lowers blood glucose levels, was demonstrated previously to promote hepatic glycogen synthesis, stabilize hepatic glycogen stores, activate glycogen synthase, inactivate glycogen phosphorylase, and inhibit glycolysis. In the present study proglycosyn was found to inhibit fatty acid synthesis, stimulate fatty acid oxidation, and lower fructose 2,6-bisphosphate levels, but to have no significant effects on cell swelling and the levels of cAMP in hepatocytes prepared from fed rats. Verapamil and atropine blocked the effects of proglycosyn on glycogen metabolism, but these compounds inhibit proglycosyn accumulation by hepatocytes. Proglycosyn stimulated phosphoprotein phosphatase activity in postmitochondrial extracts, as measured by dephosphorylation of phosphorylase a and glycogen synthase D, but this action required a very high concentration of the compound, making it unlikely to be the actual mechanism involved. It is proposed that a metabolite of proglycosyn is responsible for its metabolic effects.

Acute and chronic ethanol exposure alters the function of hippocampal kainate receptors expressed in Xenopus oocytes

The effects of acute and extended ethanol exposure on N-methyl-D-aspartate- and kainate-induced currents were examined electrophysiologically in Xenopus oocytes expressing rat hippocampal mRNA. Ethanol inhibited responses stimulated by low and high concentrations of N-methyl-D-aspartate to a similar degree. However, responses produced by low or high concentrations of kainate were differentially inhibited by ethanol. Low kainate concentration responses were much more sensitive to ethanol than high kainate concentrations (e.g., 50 mM ethanol inhibited 12.5 microM kainate responses by 45% compared to 15% inhibition of 400 microM kainate responses). In oocytes cultured in 100 mM ethanol for 1-5 days, the ethanol inhibition of maximum N-methyl-D-aspartate and kainate responses was not different from that in non-ethanol-exposed oocytes. Ethanol treatment, however, selectively decreased the ethanol sensitivity of low kainate concentration responses. Currents stimulated by N-methyl-D-aspartate or kainate were not different between control and ethanol-treated oocytes, indicating that ethanol exposure did not interfere with channel expression. The selective actions of acute and extended ethanol exposure on low kainate responses may indicate selective actions of ethanol on subtypes of kainate receptors expressed in oocytes.

Coenzyme A- and NADH-dependent esterase activity of methylmalonate semialdehyde dehydrogenase

Methylmalonate semialdehyde dehydrogenase purified to homogeneity from rat liver possesses, in addition to its coupled aldehyde dehydrogenase and CoA ester synthetic activity, the ability to hydrolyze p-nitrophenyl acetate. The following observations suggest that this activity is an active site phenomenon: (a) p-nitrophenyl acetate hydrolysis was inhibited by malonate semialdehyde, substrate for the dehydrogenase reaction; (b) p-nitrophenyl acetate was a strong competitive inhibitor of the dehydrogenase activity; (c) NAD+ and NADH activated the esterase activity; (d) coenzyme A, acceptor of acyl groups in the dehydrogenase reaction, accelerated the esterase activity; and (e) the product of the esterase reaction proceeding in the presence of coenzyme A was acetyl-CoA. These findings suggest that an S-acyl enzyme (thioester intermediate) is likely common to both the esterase reaction and the aldehyde dehydrogenase/CoA ester synthetic reaction.

Highly efficient generation of recombinant baculoviruses by enzymatically medicated site-specific in vitro recombination

We have used the Cre-lox system of bacteriophage P1 to develop a highly efficient in vitrosystem for construction of recombinant baculoviruses. A positive visual selection has been included to make identification of recombinant viral progeny rapid and straightforward. We report recombination frequencies as high as 5 x 10(7) recombinants/micrograms starting plasmid DNA and under certain conditions, up to 50% of the viral progeny are recombinants. Genes inserted into the baculovirus genome can be readily recovered in a simple one step process and re-inserted after manipulation if required. We have confirmed the structure of recovered plasmids by diagnostic restriction endonuclease digestion and the structure of recombinant viral genomes by Southern analysis. Possible uses and the significance of the system are discussed and experiments currently being done to improve it are described.

Purification, characterization, regulation and molecular cloning of mitochondrial protein kinases

The mitochondrial kinases responsible for the phosphorylation and inactivation of rat heart pyruvate dehydrogenase complex and the rat liver and heart branched-chain alpha-ketoacid dehydrogenase complexes have been purified to homogeneity. The branched-chain alpha-ketoacid dehydrogenase kinase is composed of one subunit with a molecular weight of 44 kDa; pyruvate dehydrogenase kinase has two subunits with molecular weights of 48 (alpha) and 45 kDa (beta). Proteolysis maps of branched-chain alpha-ketoacid dehydrogenase kinase and the two subunits of pyruvate dehydrogenase kinase are different, suggesting that all subunits are different entities. The alpha subunit of the rat heart pyruvate dehydrogenase kinase was selectively cleaved by chymotrypsin with concomitant loss of kinase activity, as previously shown for the bovine kidney enzyme, suggesting that the catalytic activity of pyruvate dehydrogenase kinase resides in this subunit. Polyclonal antibodies against branched-chain alpha-ketoacid dehydrogenase kinase, purified by an epitope selection method, bound only to the 44 kDa polypeptide of the branched-chain alpha-ketoacid dehydrogenase complex, substantiating that the 44 kDa protein corresponds to the kinase for this complex. Both kinases exhibited strong substrate specificity toward their respective complexes and would not inactivate heterologous complexes. The kinases possessed slightly different substrate specificities toward histones. Phosphorylation and inactivation of the branched-chain alpha-ketoacid dehydrogenase complex by its purified kinase was inhibited by alpha-chloroisocaproate and dichloroacetate, established inhibitors of the phosphorylation of the complex. cDNAs encoding the branched-chain alpha-ketoacid dehydrogenase kinase have been isolated from rat heart and rat liver lambda gt11 libraries. This represents the first successful cloning of a mitochondrial protein kinase. Preliminary data suggest that two different isoforms of the kinase may exist in different ratios in various tissues. No evidence was found for induction of the branched-chain alpha-ketoacid dehydrogenase complex nor its kinase by clofibric acid. Rather, clofibric acid is a potent inhibitor of the activity of the branched-chain alpha-ketoacid dehydrogenase kinase and this may be the molecular mechanism responsible for the myotonic effects of clofibric acid in man.

A comparative study of hemocytes from six different snails: morphology and functional aspects

Hemocytes taken from six different gastropod snails, Achatina achatina, A. fulica, Biomphalaria glabrata, Bulinus natalensis, Helix aspersa, and Lymnaea stagnalis, were compared for morphology, peroxidase activity, and, using methods developed for L. stagnalis, the ability to generate reactive oxygen inermediates upon phagocytic stimulation. Numbers of hemocytes per milliliter hemolymph and hemocytes' microscopical morphology showed some variation among the snail species. Peroxidase activity was demonstrated in all snail hemocytes except in those of B. glabrata and A. fulica. Hemocytes of all species generated superoxide upon phagocytic stimulation with zymosan (tested by superoxide dismutase-inhibitable reduction of nitroblue tetrazolium). When tested, hemocytes of A. achatina and of A. fulica displayed luminol-dependent chemiluminescence activity.

Modulation of gamma-aminobutyric acidA receptor-operated chloride channels by benzodiazepine inverse agonists is related to genetic differences in ethanol withdrawal seizure severity

To determine whether genetic differences in development of ethanol dependence are related to changes in gamma-aminobutyric acidA (GABAA) receptor function, we measured 36Cl- uptake by brain cortical membrane vesicles from withdrawal seizure prone and withdrawal seizure resistant (WSP/WSR) mice treated chronically with ethanol. Muscimol-stimulated chloride flux was not different between WSP and WSR mice before or after ethanol treatment. Also, augmentation of muscimol action by flunitrazepam or inhibition of muscimol action by the inverse agonists Ro 15-4513 (ethyl-8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5a]- [1,4]benzodiazepine-3-carboxylate) and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) was not different for ethanol-naive WSP and WSR mice. However, chronic ethanol administration enhanced the inhibitory actions of DMCM and Ro 15-4513 on membranes from WSP but not WSR mice. Conversely, chronic ethanol treatment attenuated the action of flunitrazepam on membranes from WSR but not WSP mice, suggesting that the actions of benzodiazepine agonists and inverse agonists are under separate genetic control. These genetic differences in actions of DMCM and Ro 15-4513 indicate that sensitization to benzodiazepine inverse agonists produced by chronic ethanol treatment may be related to development of withdrawal seizures and suggest that differences in the GABA/benzodiazepine receptor complex represent alleles that have segregated during the selection of the WSP/WSR mice.

A transient osmotic permeabilization method for the introduction of impermeant molecules into functional brain membrane vesicles

Mouse brain membrane vesicles (microsacs) were transiently permeabilized by hypo-osmotic shock. This permeabilization method resulted in the encapsulation of both [14C]sucrose and exogenous alkaline phosphatase. The efficiency of this method was estimated by [14C]sucrose encapsulation experiments to be approximately 81%. External membrane binding experiments with the lectin [3H]concanavalin A demonstrate that the microsacs were not inverted by permeabilization. Following permeabilization, the functional integrity of a ligand-gated ion channel, the GABAA receptor complex, was investigated. Muscimol-stimulated 36Cl-uptake experiments show that this receptor retains its functional properties including blockade by the receptor antagonist bicuculline and potentiation by the allosteric modulators flunitrazepam and pentobarbital. The osmotic permeabilization technique described here provides several advantages over other permeabilization methods. These advantages include a high trapping efficiency, the encapsulation of not only small solutes but large membrane impermeant compounds such as enzymes and the functional preservation of at least one transmembrane protein. Furthermore, this method does not require specialized equipment and does not result in large, permanent holes in the plasma membrane.

A general method for quantitative PCR analysis of mRNA levels for members of gene families: application to GABAA receptor subunits

We have developed a sensitive, PCR-based method for quantitating changes in mRNA levels of members of gene families. In this approach, total mRNA is converted to cDNA and then PCR is carried out on family members simultaneously, using primers derived from regions conserved among family members. This is followed by gel electrophoresis and blotting of the product to filters. The level of expression of individual family members is determined by separate hybridizations using probes unique for each member and derived from sequences between the PCR primers. In this manner the same aliquot of mRNA, the same reverse transcriptase reaction, PCR, gel electrophoresis, and denaturation and blotting are used for analysis of each family member. Thus, experimental variation is minimized, and changes in mRNA levels of family members relative to one another can be monitored with precision. In addition, if a family member is known not to change as a result of the treatment employed, this mRNA can be used to normalize the data from other members and thereby allow individual variations to be quantitated. We have applied this approach to members of the GABAA receptor subunit gene family and studied effects of chronic ethanol treatment on mRNAs corresponding to several GABAA receptor subunits.

Chronic ethanol treatment alters brain levels of gamma-aminobutyric acidA receptor subunit mRNAs: relationship to genetic differences in ethanol withdrawal seizure severity

Chronic ethanol treatment is known to alter the function of the gamma-aminobutyric acidA (GABAA) benzodiazepine receptor complex. To determine if genetic differences in development of ethanol dependence are related to expression of GABAA receptor subunits, we measured whole brain levels of mRNA for the alpha 1, alpha 3, alpha 6, gamma 2s, gamma 2L, and gamma 3 receptor subunits in withdrawal seizure-prone and -resistant (WSP and WSR, respectively) mice fed an ethanol-containing liquid diet or a control diet. Brain poly(A)+ RNA was converted to cDNA and amplified by the polymerase chain reaction using primers conserved among GABAA receptor subunits. Quantification was carried out by densitometric analysis of Southern blots generated using subunit-specific probes. Chronic ethanol treatment decreased the content of alpha 1 mRNA in WSP but not WSR mice and decreased the content of alpha 6 mRNA in WSR but not WSP mice. The content of gamma 3 mRNA was increased by chronic ethanol in both lines. In untreated mice, the WSP line had lower levels of alpha 3 and alpha 6 mRNA than the WSR line. Thus, a decrease in the content of alpha 1 mRNA is most clearly linked with development of withdrawal signs, although the amounts of alpha 6 and alpha 3 may also be important in the genetic differences between WSP and WSR mice. In contrast, levels of mRNA for gamma 2S and gamma 2L subunits do not appear to be altered in ethanol dependence.

The effect of ligand binding on the proteolytic pattern of methylmalonate semialdehyde dehydrogenase

Native rat liver methylmalonate semialdehyde dehydrogenase was proteolyzed by lysylendopeptidase C, chymotrypsin, and trypsin to generate different cleavage fragments of molecular masses: 50, 8, 55, 44, 39, 53, 45, and 40 kDa. A proteolytic cleavage map of MMSDH was constructed based on sequencing data and a comparison of appearance and degradation rates of the different protein fragments as shown by SDS-PAGE. NAD+ was highly effective as a protector against proteolysis in both the N-terminal and the C-terminal parts of the intact enzyme. NADH did not efficiently protect the intact enzyme; however, it stabilized proteolytic fragment L50 from further degradation. This suggests that the NAD(+)-binding domain is not destroyed by cleavage of the N-terminal part of MMSDH. CoA had no effect on the proteolytic cleavage patterns of MMSDH. However, CoA esters reduced the protective effect of NAD+ with an order of effectiveness of acetyl-CoA greater than propionyl-CoA greater than butyryl-CoA. p-Nitrophenyl acetate, substrate for esterase activity by the enzyme, partially prevented the protective effect of NAD+ against proteolysis. These results suggest that S-acylation of the enzyme prevents a stabilizing conformational change induced in MMSDH by NAD+ binding.

Immunochemical identification of branched-chain 2-oxo acid dehydrogenase kinase

Branched-chain 2-oxo acid dehydrogenase kinase was characterized using anti-kinase polyclonal antibodies. The antibodies were purified from rabbit antiserum by an epitope selection method. The antibodies bound only to a 44 kDa polypeptide in the dehydrogenase-kinase complex and inhibited the kinase activity, substantiating that the 44 kDa polypeptide is the catalytic subunit of the kinase. The purified liver dehydrogenase-kinase complex, but not either the purified heart complex or the partially purified liver complex, contained 2 additional polypeptides of lower molecular weight which also reacted with the anti-kinase antibodies, suggesting that the liver kinase is subject to proteolytic degradation during purification.

Effects of various secretagogues on [Ca2+]i in cultured human nasal epithelial cells

Recent studies conducted on cultured canine tracheal cells have suggested that a complex relationship exists between cAMP and Ca2+ in the control of electrolyte secretion. The goal of this study was to determine if the Ca2+ second messenger system functions in a similar fashion in cultured human nasal epithelial cells, a tissue in which control of electrolyte secretion is known to be disrupted in the genetic disease, cystic fibrosis. Human nasal epithelial tissue was obtained as a by-product of surgery and put into monolayer cell culture. After 4-5 days in culture, cells were loaded with fura-2 and intracellular free Ca2+ measured as previously reported. We found that bradykinin increased intracellular free Ca2+ in all cells tested, whereas isoproterenol increased intracellular free Ca2+ in only half the cells tested, suggesting that more than one transporting cell type may be present in this tissue. Epinephrine and prostaglandin E2 had no effect on intracellular free Ca2+. We found that the voltage-sensitive Ca2+ channel blocker verapamil had no effect on the bradykinin-induced change in intracellular free Ca2+. Removal of Ca2+ from the bathing saline only slightly attenuated the increase in intracellular free Ca2+ that resulted from stimulation with bradykinin. We conclude that the source of the increased intracellular free Ca2+, observed during stimulation with secretagogues, was primarily intracellular stores.

Cyclic AMP-dependent protein kinase decreases gamma-aminobutyric acidA receptor-mediated 36Cl- uptake by brain microsacs

The effect of cyclic AMP (cAMP)-dependent protein phosphorylation on gamma-aminobutyric acidA (GABAA) receptor function was examined using isolated brain membrane vesicles (microsacs). Muscimol-stimulated 36Cl- uptake was studied in mouse brain microsacs permeabilized to introduce the catalytic subunit of cAMP-dependent protein kinase (PKA). At both submaximal and maximally effective concentrations of muscimol, PKA inhibited muscimol-stimulated 36Cl- uptake by approximately 25%. In parallel experiments, PKA and [gamma-32P]ATP were introduced into the microsacs, and we attempted to immunoprecipitate the entire GABAA receptor complex, under nondenaturing conditions, using an anti-alpha 1-subunit antibody. Data from such experiments show that PKA increases the phosphorylation of several microsac proteins, including a 66-kDa polypeptide specifically immunoprecipitated with the GABAA receptor anti-alpha 1 subunit antibody. Phosphopeptide mapping of the 66-kDa polypeptide demonstrated a 14-kDa fragment similar to that obtained with the purified, PKA-phosphorylated GABAA receptor. These results provide evidence that the catalytic subunit of PKA inhibits the function of brain GABAA receptors and demonstrate that this functional change is concomitant with an increase in protein phosphorylation.

Purification and comparative study of the kinases specific for branched chain alpha-ketoacid dehydrogenase and pyruvate dehydrogenase

Rat heart branched chain alpha-ketoacid dehydrogenase kinase (BCKDH kinase) and pyruvate dehydrogenase kinase (PDH kinase) were purified from their respective complexes to apparent homogeneity. BCKDH kinase consisted of one subunit with molecular weight 44,000-45,000 Da, whereas PDH kinase consisted of two subunits with molecular weight 48,000 Da (alpha) and 45,000 Da (beta) as previously shown for the bovine kidney enzyme (Stepp et al., 1983, J. Biol. Chem. 258, 9454-9458). Proteolysis maps of BCKDH kinase and the two subunits of PDH kinase were different, suggesting that all subunits are different entities. The alpha subunit of the rat heart PDH kinase could be cleaved selectively by chymotrypsin with concomitant loss of kinase activity, as previously shown for the bovine kidney enzyme, suggesting that the catalytic activity of PDH kinase resides in the alpha subunit. The beta subunit appeared to be a different entity unique to the PDH kinase. Both kinases exhibited marked substrate specificity toward their respective complexes and would not inactivate heterologous complexes. The kinases possessed slightly different substrate specificity toward histones. BCKDH kinase preferentially phosphorylated histones in the order f1 greater than f2B much greater than f2A much greater than f3. The relative order for PDH kinase was the same, but f2A and f3 were considerably better substrates than they were for BCKDH kinase. These observations suggest that the kinases have different requirements for the structure of the protein at their phosphorylation sites.

Cerebellar GABAB receptors modulate function of GABAA receptors

Interactions between GABAA and GABAB receptors were studied using muscimol-stimulated uptake of 36Cl- by membrane vesicles from mouse cerebellum. Baclofen inhibited muscimol-stimulated 36Cl- uptake and this action was more pronounced with longer flux times (30 vs. 3 s) and after predesensitization of GABAA receptors. Baclofen also inhibited 36Cl- flux by cortical membranes but was more effective with cerebellar preparations. The action of baclofen was stereoselective, calcium-dependent, and blocked by the GABAB receptor antagonist 2-OH-saclofen. It was mimicked by GTP-gamma-S but not by GDP-beta-S, which suggests that baclofen may be acting via a G protein. The action of baclofen was inhibited by U73122, an inhibitor of phospholipase C. However, the potassium channel blockers tetraethylammonium or Ba2+ did not affect the action of baclofen. The results show that activation of GABAB receptors can inhibit the function of GABAA receptors and suggest that this action involves either a nondesensitizing subtype of GABAA receptor or the rate or recycling of desensitized to nondesensitized receptors. We speculate that this action of baclofen results from activation of phospholipase C and phosphorylation of a subtype of GABAA receptor by protein kinase C.

Ethanol sensitivity of the GABAA receptor expressed in Xenopus oocytes requires 8 amino acids contained in the gamma 2L subunit

Expression of brain mRNA or cRNAs in Xenopus oocytes was used to determine what subunits of the GABAA receptor are required for modulation by barbiturates, benzodiazepines, and ethanol. Mouse brain mRNA was hybridized with antisense oligonucleotides complementary to sequences unique to specific subunits and injected into oocytes. Antisense oligonucleotides to the alpha 1, beta 1, gamma 1, gamma 2S + 2L, gamma 2L, or gamma 3 subunits did not alter GABA action or enhancement by pentobarbital. Action of diazepam was prevented by antisense oligonucleotides to gamma 2S + 2L and reduced by antisense sequences to gamma 2L, but was not affected by the other oligonucleotides. Ethanol enhancement of GABA action was prevented only by antisense oligonucleotides to gamma 2L (which differs from gamma 2S by the addition of 8 amino acids). Expression of either the alpha 1 beta 1 gamma 2S or the alpha 1 beta 1 gamma 2L subunit cRNA combination in oocytes resulted in GABA responses that were enhanced by diazepam or pentobarbital, but only the combination containing the gamma 2L subunit was affected by ethanol.