Salmonella typhimurium histidinol dehydrogenase: complete reaction stereochemistry and active site mapping.

The stereochemistry of the L-histidinol dehydrogenase reaction was determined to be R at NAD for both steps, confirming previous results with a fungal extract [Davies, D., Teixeira, A., & Kenworthy, P. (1972) Biochem. J. 127, 335-343]. NMR analysis of monodeuteriohistidinols produced by histidinol/NADH exchange reactions arising via reversal of the alcohol oxidation reaction indicated a single stereochemistry at histidinol for that step. Comparison of vicinal coupling values of the exchange products with those of L-alaninol and a series of (S)-2-amino-1-alcohols allowed identification of the absolute stereochemistry of monodeuteriohistidinols and showed that histidinol dehydrogenase removes first the pro-S then the pro-R hydrogens of substrate histidinol. The enzyme stereochemistry was confirmed by isotope effects for monodeuteriohistidinols as substrates for the pro-R-specific dehydrogenation catalyzed by liver alcohol dehydrogenase. Active site mapping was undertaken to investigate substrate-protein interactions elsewhere in the histidinol binding site. Critical binding regions are the side-chain amino group and the imidazole ring, whose methylation at the 1- or 2-position caused severe decreases in binding affinity. Use of alternative substrates further clarified active site interactions with the substrate. Compounds in which the alpha-amino group was replaced by chloro, bromo, or hydrogen substituents were not substrates of the overall reaction at 1/10,000 the normal rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic mechanism of histidinol dehydrogenase: histidinol binding and exchange reactions.

Salmonella typhimurium histidinol dehydrogenase produces histidine from the amino alcohol histidinol by two sequential NAD-linked oxidations which form and oxidize a stable enzyme-bound histidinaldehyde intermediate. The enzyme was found to catalyze the exchange of 3H between histidinol and [4(R)-3H]NADH and between NAD and [4(S)-3H]NADH. The latter reaction proceeded at rates greater than kcat for the net reaction and was about 3-fold faster than the former. Histidine did not support an NAD/NADH exchange, demonstrating kinetic irreversibility in the second half-reaction. Specific activity measurements on [3H]histidinol produced during the histidinol/NADH exchange reaction showed that only a single hydrogen was exchanged between the two reactants, demonstrating that under the conditions employed this exchange reaction arises only from the reversal of the alcohol dehydrogenase step and not the aldehyde dehydrogenase reaction. The kinetics of the NAD/NADH exchange reaction demonstrated a hyperbolic dependence on the concentration of NAD and NADH when the two were present in a 1:2 molar ratio. The histidinol/NADH exchange showed severe inhibition by high NAD and NADH under the same conditions, indicating that histidinol cannot dissociate directly from the ternary enzyme-NAD-histidinol complex; in other words, the binding of substrate is ordered with histidinol leading. Binding studies indicated that [3H]histidinol bound to 1.7 sites on the dimeric enzyme (0.85 site/monomer) with a KD of 10 microM. No binding of [3H]NAD or [3H]NADH was detected. The nucleotides could, however, displace histidinol dehydrogenase from Cibacron Blue-agarose.(ABSTRACT TRUNCATED AT 250 WORDS)

Deficits in the control of food intake after hypothalamic paraventricular nucleus lesions.

Noradrenergic mechanisms of the hypothalamic paraventricular nucleus (PVN) have been shown to play an important role in the stimulation of feeding To determine the influence of this nucleus in monitoring and controlling responses to physiological and pharmacological challenges, PVN electrolytic lesion rats were tested for their behavioral responsiveness to agents known to affect the alpha-2 noradrenergic system as well as release of corticosterone, and to short- and long-term periods of food deprivation. Discrete lesions of the PVN produced enhanced feeding, particularly of carbohydrate, in freely-feeding rats maintained on a macronutrient self-selection paradigm. Lesion rats demonstrated a behavioral deficit in food intake regulation (a decrease in carbohydrate ingestion) in response to 5-hr and 24-hr fasts, showed a disturbance in circadian feeding, and exhibited a dramatic decrease in circulating corticosterone. However, feeding in response to 2-deoxy-D-glucose and insulin remained intact, suggesting that noradrenergic receptors within the PVN are not involved in the mediation of glucoprivic-induced feeding.