Molecular cloning of the mature NAD(+)-dependent succinic semialdehyde dehydrogenase from rat and human. cDNA isolation, evolutionary homology, and tissue expression

Plant succinic semialdehyde dehydrogenase. Cloning, purification, localization in mitochondria, and regulation by adenine nucleotides

Succinic semialdehyde dehydrogenase (SSADH) is one of three enzymes constituting the gamma-aminobutyric acid shunt. We have cloned the cDNA for SSADH from Arabidopsis, which we designated SSADH1. SSADH1 cDNA encodes a protein of 528 amino acids (56 kD) with high similarity to SSADH from Escherichia coli and human (>59% identity). A sequence similar to a mitochondrial protease cleavage site is present 33 amino acids from the N terminus, indicating that the mature mitochondrial protein may contain 495 amino acids (53 kD). The native recombinant enzyme and the plant mitochondrial protein have a tetrameric molecular mass of 197 kD. Fractionation of plant mitochondria revealed its localization in the matrix. The purified recombinant enzyme showed maximal activity at pH 9.0 to 9.5, was specific for succinic semialdehyde (K(0.5) = 15 microM), and exclusively used NAD+ as a cofactor (Km = 130 +/- 77 microM). NADH was a competitive inhibitor with respect to NAD+ (Ki = 122 +/- 86 microM). AMP, ADP, and ATP inhibited the activity of SSADH (Ki = 2.5-8 mM). The mechanism of inhibition was competitive for AMP, noncompetitive for ATP, and mixed competitive for ADP with respect to NAD+. Plant SSADH may be responsive to mitochondrial energy charge and reducing potential in controlling metabolism of gamma-aminobutyric acid.

Identification of a mutation cluster in mevalonate kinase deficiency, including a new mutation in a patient of Mennonite ancestry

Mevalonate kinase (MKase) deficiency (MKD) is a rare autosomal recessive disorder in the pathway of cholesterol and nonsterol isoprenoid biosynthesis. Thus far, two disease-causing missense alleles have been identified, N301T and A334T. We report four additional mutations associated with MKD: L264F, T243I, L265P, and I268T, the last found in a patient of Mennonite ancestry. Electrophoretic analysis of bacterially expressed wild-type and mutant MKase indicated that I268T and T243I mutants produced normal or somewhat reduced amounts of MKase protein; conversely, L264F and L265P mutations resulted in considerably decreased, or absent, MKase protein. Immunoblot analysis of MKase from all patients suggested that the MKase polypeptide was grossly intact and produced in amounts comparable to control levels. Three mutations resulted in significantly diminished MKase enzyme activity (<2%), whereas the I268T allele yielded approximately 20% residual enzyme activity. Our results should allow more-accurate identification of carriers and indicate a mutation "cluster" within amino acids 240-270 of the mature MKase polypeptide.

Relationships within the aldehyde dehydrogenase extended family

One hundred-forty-five full-length aldehyde dehydrogenase-related sequences were aligned to determine relationships within the aldehyde dehydrogenase (ALDH) extended family. The alignment reveals only four invariant residues: two glycines, a phenylalanine involved in NAD binding, and a glutamic acid that coordinates the nicotinamide ribose in certain E-NAD binary complex crystal structures, but which may also serve as a general base for the catalytic reaction. The cysteine that provides the catalytic thiol and its closest neighbor in space, an asparagine residue, are conserved in all ALDHs with demonstrated dehydrogenase activity. Sixteen residues are conserved in at least 95% of the sequences; 12 of these cluster into seven sequence motifs conserved in almost all ALDHs. These motifs cluster around the active site of the enzyme. Phylogenetic analysis of these ALDHs indicates at least 13 ALDH families, most of which have previously been identified but not grouped separately by alignment. ALDHs cluster into two main trunks of the phylogenetic tree. The largest, the "Class 3" trunk, contains mostly substrate-specific ALDH families, as well as the class 3 ALDH family itself. The other trunk, the "Class 1/2" trunk, contains mostly variable substrate ALDH families, including the class 1 and 2 ALDH families. Divergence of the substrate-specific ALDHs occurred earlier than the division between ALDHs with broad substrate specificities. A site on the World Wide Web has also been devoted to this alignment project.

Cloning of a rat brain succinic semialdehyde reductase involved in the synthesis of the neuromodulator gamma-hydroxybutyrate

The gamma-hydroxybutyrate biosynthetic enzyme succinic semialdehyde reductase (SSR) was purified to homogeneity from rat brain. Peptides were generated by tryptic cleavage and sequenced. PCR primers were designed from the amino acid sequences of two of the peptides showing a similarity (75-85%) to a mitochondrial aldehyde dehydrogenase. A PCR-amplified DNA fragment was generated from recombinant plasmids prepared by a mass excision procedure from a rat hippocampal cDNA library and used as a probe to screen this cDNA library. One cDNA of 1341 bp had an open reading frame encoding a protein of 447 residues with a deduced molecular mass of 47967 Da. The enzyme was expressed in Escherichia coli. Immunoblotting analysis revealed the existence of a protein with the same electrophoretic mobility as the SSR purified from rat brain and with an estimated molecular mass of 45 kDa. Northern blot experiments showed that this enzyme was not expressed in the kidney or in the liver. In the brain tissue, a single but rather broad band was labelled under high stringency conditions, suggesting the presence of more than one messenger species coding for SSR. Hybridization in situ performed on brain tissue slices showed specific labelling of the hippocampus, the upper cortex layer, the thalamus, the substantia nigra, the cerebellum, the pons medulla and the olfactory tract. The recombinant enzyme showed catalytic properties similar to those of the SSR purified from rat brain, particularly in regard to its substrate affinities and Ki for inhibition by phthalaldehydic acid. Valproic acid did not inhibit the cloned SSR. This enzyme had 20-35% identity in highly conserved regions involved in NADPH binding with four other proteins belonging to the aldo-oxo reductase family.

Two exon-skipping mutations as the molecular basis of succinic semialdehyde dehydrogenase deficiency (4-hydroxybutyric aciduria)

Succinic semialdehyde dehydrogenase (SSADH) deficiency, a rare metabolic disorder of 4-aminobutyric acid degradation, has been identified in approximately 150 patients. Affected individuals accumulate large quantities of 4-hydroxybutyric acid, a compound with a wide range of neuropharmacological activities, in physiological fluids. As a first step in beginning an investigation of the molecular genetics of SSADH deficiency, we have utilized SSADH cDNA and genomic sequences to identify two point mutations in the SSADH genes derived from four patients. These mutations, identified by standard methods of reverse transcription, PCR, dideoxy-chain termination, and cycle sequencing, alter highly conserved sequences at intron/exon boundaries and prevent the RNA-splicing apparatus from properly recognizing the normal splice junction. Each family segregated a mutation in a different splice site, resulting in exon skipping and, in one case, a frameshift and premature termination and, in the other case, an in-frame deletion in the resulting protein. Family members, including parents and siblings of these patients, were shown to be heterozygotes for the splicing abnormality, providing additional evidence for autosomal recessive inheritance. Our results provide the first evidence that 4-hydroxybutyric aciduria, resulting from SSADH deficiency, is the result of genetic defects in the human SSADH gene.

Molecular cloning of human phosphomevalonate kinase and identification of a consensus peroxisomal targeting sequence

Two overlapping cDNAs which encode human liver phosphomevalonate kinase (PMKase) were isolated. The human PMKase cDNAs predict a 191-amino acid protein with a molecular weight of 21,862, consistent with previous reports for mammalian PMKase (Mr = 21,000-22,500). Further verification of the clones was obtained by expression of PMKase activity in bacteria using a composite 1024-base pair cDNA clone. Northern blot analysis of several human tissues revealed a doublet of transcripts at approximately 1 kilobase (kb) in heart, liver, skeletal muscle, kidney, and pancreas and lower but detectable transcript levels in brain, placenta, and lung. Analysis of transcripts from human lymphoblasts subcultured in lipid-depleted sera (LDS) and LDS supplemented with lovastatin indicated that PMKase gene expression is subject to regulation by sterol at the level of transcription. Southern blotting indicated that PMKase is a single copy gene covering less than 15 kb in the human genome. The human PMKase amino acid sequence contains a consensus peroxisomal targeting sequence (PTS-1), Ser-Arg-Leu, at the C terminus of the protein. This is the first report of a cholesterol biosynthetic protein which contains a consensus PTS-1, providing further evidence for the concept that early cholesterol and nonsterol isoprenoid biosynthesis may occur in the peroxisome.

Mouse microsomal Class 3 aldehyde dehydrogenase: AHD3 cDNA sequence, inducibility by dioxin and clofibrate, and genetic mapping

We have cloned and sequenced the mouse AHD3 cDNA, which codes for the Class 3 microsomal aldehyde dehydrogenase (ALDH3m). The cDNA is 2,997 bp in length excluding the poly(A)+ tail, and has 5' and 3' non-translated regions of 113 bp and 1,429 bp, respectively. The deduced amino acid sequence consists of 484 amino acids, including the first methionine (Mr = 53,942), and contains a hydrophobic segment at the carboxyl terminus which is the putative membrane anchor. The mouse AHD3 protein was found to be: 95% similar to the rat microsomal ALDH3m protein, 65% identical to the mouse, rat and human cytosolic ALDH3c protein, and <28% similar to the rat Class 1 and Class 2 ALDH and methylmalonate-semialdehyde dehydrogenase proteins. Southern hybridization analysis of mouse cDNA probed with the full-length AHD3 cDNA revealed that the Ahd3 gene likely spans less than a total of 25 kb. The mouse Ahd3 gene is very tightly linked to the Ahd4 gene on chromosome 11. Mouse AHD3 mRNA levels are increased by dioxin in mouse Hepa-1c1c7 hepatoma wild-type (wt) cells but not in the Ah receptor nuclear translocator (ARNT)-defective (c4) mutant line, indicating that the induction process is mediated by the Ah (aromatic hydrocarbon) dioxin-binding receptor. AHD3 mRNA levels are also inducible by clofibrate in both the wt and c4 lines. AHD3 mRNA levels are not elevated in the CYP1A1 metabolism-deficient c37 mutant line or as part of the oxidative stress response found in the untreated 14CoS/14CoS mouse cell line. These data indicate that, although inducible by dioxin, the Ahd3 gene does not qualify as a member of the aromatic hydrocarbon [Ah] gene battery.