Germ cell apoptosis in the testes of normal stallions

Apoptosis in testicular germ cells has been demonstrated in many mammalian species. However, little is known about the stallion (Equus caballus) and rates of apoptosis during spermatogenesis. Morphological and biochemical features of apoptosis reported in other species were used to confirm that the TdT-mediated dUTP Nick end labeling (TUNEL) assay is an acceptable method for identification and quantification of apoptotic germ cells in histological tissue sections from stallion testis. Seminiferous tubules from eight stallions with normal testis size and semen quality were evaluated according to stage of seminiferous epithelium to determine the germ cell types and stages where apoptosis most commonly occurs. Spermatogonia and spermatocytes were the most common germ cell types labeled by the TUNEL assay. A low rate of round and elongated spermatids were labeled by the TUNEL assay. Mean numbers of TUNEL-positive germ cells per 100 Sertoli cell nuclei were highest in stages IV (15.5 +/- 1.0) and V (13.5 +/- 1.1) of the seminiferous epithelial cycle (P < 0.001). An intermediate level of apoptosis was detected in stage VI (P < 0.02). These stages (IV-VI) correspond to meiotic divisions of primary spermatocytes and mitotic proliferation of B1 and B2 spermatogonia. Establishing basal levels of germ cell apoptosis is a critical step towards understanding fertility and the role of apoptosis in regulating germ cell numbers during spermatogenesis.     

Relationships between the flexed-arm hang and select measures of muscular fitness

The flexed-arm hang (FAH) has been used to assess arm and shoulder girdle strength for 35 years despite little evidence to support its use. The purpose of this study was to determine what muscular fitness component, if any, was related to the FAH. The traditional overgrip chin-above-bar test and 5 different variations were compared with absolute strength (1 repetition maximum [1RM] lat pull down), relative strength (1RM.mass(-1)), and muscle endurance (repetitions to failure at 70% of the 1RM). Sixty college-age women volunteered for the study. Relationships were examined using Pearson Product Moment Correlation. No significant relationship was found between any of the FAH variations and absolute strength or muscle endurance; however, all FAH variations correlated significantly with relative strength (1RM.mass(-1)). The strongest relationship was with the undergrip FAH timed to 90 degrees of elbow extension (r = 0.72). Investigators concluded that the FAH is a test of weight-relative muscular strength and appears unrelated to absolute strength or muscle endurance.     

Crystal structure of mycothiol synthase (Rv0819) from Mycobacterium tuberculosis shows structural homology to the GNAT family of N-acetyltransferases

Mycothiol is the predominant low-molecular weight thiol produced by actinomycetes, including Mycobacterium tuberculosis. The last reaction in the biosynthetic pathway for mycothiol is catalyzed by mycothiol synthase (MshD), which acetylates the cysteinyl amine of cysteine-glucosamine-inositol (Cys-GlcN-Ins). The crystal structure of MshD was determined in the presence of coenzyme A and acetyl-CoA. MshD consists of two tandem-repeated domains, each exhibiting the Gcn5-related N-acetyltransferase (GNAT) fold. These two domains superimpose with a root-mean-square deviation of 1.7 A over 88 residues, and each was found to bind one molecule of coenzyme, although the binding sites are quite different. The C-terminal domain has a similar active site to many GNAT members in which the acetyl group of the coenzyme is presented to an open active site slot. However, acetyl-CoA bound to the N-terminal domain is buried, and is apparently not positioned to promote acetyl transfer. A modeled substrate complex indicates that Cys-GlcN-Ins would only fill a portion of a negatively charged channel located between the two domains. This is the first structure determined for an enzyme involved in the biosynthesis of mycothiol.     

Mycobacterium tuberculosis ketopantoate hydroxymethyltransferase: tetrahydrofolate-independent hydroxymethyltransferase and enolization reactions with alpha-keto acids

The panB gene that encodes ketopantoate hydroxymethyltransferase has been cloned from Mycobacterium tuberculosis, expressed, and purified to homogeneity. 1H NMR spectroscopy was used to determine the rate of (i) tetrahydrofolate-independent hydroxymethyltransferase chemistry between formaldehyde and alpha-ketoisovalerate and (ii) deuterium exchange in the methylenetetrahydrofolate-independent enolization of alpha-ketoisovalerate and other alpha-keto acids, catalyzed by PanB. These studies have demonstrated that substrate enolization by PanB is divalent metal-dependent with a preference of Mg2+ > Zn2+ > Co2+ > Ni2+ > Ca2+. The rate of enolization is pH-dependent with optimal activity in the range of 7.0-7.5. The pH profile was bell-shaped, depending on the ionization state of two ionizable groups with apparent pK values of 6.2 and 8.3. Enolization and isotope exchange occurs with some alpha-keto acids (e.g., pyruvate and alpha-ketobutyrate), resulting in the complete exchange of all beta-hydrogens. Enzyme-catalyzed enolization and isotope exchange occur with other long-chain and branched alpha-keto acids, resulting in the stereospecific exchange of only one of the beta-hydrogen atoms. These results are discussed in the context of steric restrictions present in the enzyme active site and the stereochemistry of base-catalyzed isotope exchange.     

Positional isotope exchange analysis of the pantothenate synthetase reaction

Pantothenate synthetase from Mycobacterium tuberculosis catalyzes the formation of pantothenate from ATP, D-pantoate, and beta-alanine. The formation of a kinetically competent pantoyl-adenylate intermediate was established by the observation of a positional isotope exchange (PIX) reaction within (18)O-labeled ATP in the presence of d-pantoate. When [betagamma-(18)O(6)]-ATP was incubated with pantothenate synthetase in the presence of d-pantoate, an (18)O label gradually appeared in the alphabeta-bridge position from both the beta- and the gamma-nonbridge positions. The rates of these two PIX reactions were followed by (31)P NMR spectroscopy and found to be identical. These results are consistent with the formation of enzyme-bound pantoyl-adenylate and pyrophosphate upon the mixing of ATP, D-pantoate, and enzyme. In addition, these results require the complete torsional scrambling of the two phosphoryl groups of the labeled pyrophosphate product. The rate of the PIX reaction increased as the D-pantoate concentration was elevated and then decreased to zero at saturating levels of D-pantoate. These inhibition results support the ordered binding of ATP and D-pantoate to the enzyme active site. The PIX reaction was abolished with the addition of pyrophosphatase; thus, PP(i) must be free to dissociate from the active site upon formation of the pantoyl-adenylate intermediate. The PIX reaction rate diminished when the concentrations of ATP and D-pantoate were held constant and the concentration of the third substrate, beta-alanine, was increased. This observation is consistent with a kinetic mechanism that requires the binding of beta-alanine after the release of pyrophosphate from the active site of pantothenate synthetase. Positional isotope exchange reactions have therefore demonstrated that pantothenate synthetase catalyzes the formation of a pantoyl-adenylate intermediate upon the ordered addition of ATP and pantoate.     

Catalysis of diaphorase reactions by Mycobacterium tuberculosis lipoamide dehydrogenase occurs at the EH4 level

Lipoamide dehydrogenase catalyzes the reversible NAD(+)-dependent oxidation of the dihydrolipoyl cofactors that are covalently attached to the acyltransferase components of the pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and glycine reductase multienzyme complexes. It contains two redox centers: a tightly, but noncovalently, bound FAD and an enzymic disulfide, each of which can accommodate two electrons. In the two-electron-reduced enzyme (EH(2)), the disulfide is reduced while the FAD cofactor is oxidized. In the four-electron-reduced enzyme (EH(4)), both redox centers are reduced. Lipoamide dehydrogenase can also catalyze the NADH-dependent reduction of alternative electron acceptors such as 2,6-dichlorophenolindophenol, ferricyanide, quinones, and molecular oxygen (O(2)). To determine the mechanism of these "diaphorase" reactions, we generated the EH(2) and EH(4) forms of Mycobacterium tuberculosis lipoamide dehydrogenase and rapidly mixed these enzyme forms with d,l-lipoylpentanoate, 2,6-dimethyl-1,4-benzoquinone, and O(2), in a stopped-flow spectrophotometer at pH 7.5 and 4 degrees C. EH(2) reduced d,l-lipoylpentanoate >/=100 times faster than EH(4) did. Conversely, EH(4) reduced 2,6-dimethyl-1,4-benzoquinone and molecular oxygen 90 and 40 times faster than EH(2), respectively. Comparison of the rates of reduction of the above substrates by EH(2) and EH(4) with their corresponding steady-state kinetic parameters for kinetic competence leads to the conclusion that reduction of lipoyl substrates occurs with EH(2) while reduction of diaphorase substrates occurs with EH(4).     

Substrate specificity and kinetic isotope effect analysis of the Eschericia coli ketopantoate reductase

Ketopantoate reductase (EC 1.1.1.169), an enzyme in the pantothenate biosynthetic pathway, catalyzes the NADPH-dependent reduction of alpha-ketopantoate to form D-(-)-pantoate. The enzyme exhibits high specificity for ketopantoate, with V and V/K for ketopantoate being 5- and 365-fold higher than those values for alpha-ketoisovalerate and 20- and 648-fold higher than those values for alpha-keto-beta-methyl-n-valerate, respectively. For pyridine nucleotides, V/K for beta-NADPH is 3-500-fold higher than that for other nucleotide substrates. The magnitude of the primary deuterium kinetic isotope effects on V and V/K varied substantially when different ketoacid and pyridine nucleotide substrates were used. The small primary deuterium kinetic isotope effects observed using NADPH and NHDPH suggest that the chemical step is not rate-limiting, while larger primary deuterium isotope effects were observed for poor ketoacid and pyridine nucleotide substrates, indicating that the chemical reaction has become partially or completely rate-limiting. The pH dependence of (D)V using ketopantoate was observed to vary from a value of 1.1 at low pH to a value of 2.5 at high pH, while the magnitude of (D)V/K(NADPH) and (D)V/K(KP) were pH-independent. The value of (D)V is large and pH-independent when alpha-keto-beta-methyl-n-valerate was used as the ketoacid substrate. Solvent kinetic isotope effects of 2.2 and 1.2 on V and V/K, respectively, were observed with alpha-keto-beta-methyl-n-valerate. Rapid reaction analysis of NADPH oxidation using ketopantoate showed no "burst" phase, suggesting that product-release steps are not rate-limiting and the cause of the small observed kinetic isotope effects with this substrate pair. Large primary deuterium isotope effects on V and V/K using 3-APADPH in steady-state experiments, equivalent to the isotope effect observed in single turnover studies, suggests that chemistry is rate-limiting for this poorer reductant. These results are discussed in terms of a kinetic and chemical mechanism for the enzyme.     

The three-dimensional structures of the Mycobacterium tuberculosis dihydrodipicolinate reductase-NADH-2,6-PDC and -NADPH-2,6-PDC complexes. Structural and mutagenic analysis of relaxed nucleotide specificity

Dihydrodipicolinate reductase (DHPR) catalyzes the reduced pyridine nucleotide-dependent reduction of the alpha,beta-unsaturated cyclic imine, dihydrodipicolinate, to generate tetrahydrodipicolinate. This enzyme catalyzes the second step in the bacterial biosynthetic pathway that generates meso-diaminopimelate, a component of bacterial cell walls, and the amino acid L-lysine. The Mycobacterium tuberculosis dapB-encoded DHPR has been cloned, expressed, purified, and crystallized in two ternary complexes with NADH or NADPH and the inhibitor 2,6-pyridinedicarboxylate (2,6-PDC). The structures have been solved using molecular replacement strategies, and the DHPR-NADH-2,6-PDC and DHPR-NADPH-2,6-PDC complexes have been refined against data to 2.3 and 2.5 A, respectively. The M. tuberculosis DHPR is a tetramer of identical subunits, with each subunit composed of two domains connected by two flexible hinge regions. The N-terminal domain binds pyridine nucleotide, while the C-terminal domain is involved in both tetramer formation and substrate/inhibitor binding. The M. tuberculosis DHPR uses NADH and NADPH with nearly equal efficiency based on V/K values. To probe the nature of this substrate specificity, we have generated two mutants, K9A and K11A, residues that are close to the 2'-phosphate of NADPH. These two mutants exhibit decreased specificity for NADPH by factors of 6- and 30-fold, respectively, but the K11A mutant exhibits 270% of WT activity using NADH. The highly conserved structure of the nucleotide fold may permit other enzyme's nucleotide specificity to be altered using similar mutagenic strategies.