Inhibition of histidine decarboxylase by benzyl and aliphatic aminooxyamines

Transition-state analogues as inhibitors of L-dopa decarboxylase

1. A series of compounds has been prepared which are analogues of the transition state of the reaction catalysed by L-dopa decarboxylase (EC 4.1.1.28). 2. These compounds are reduced adducts of the substrate (L-dopa) and coenzyme (pyridoxal phosphate), as well as analogues of these substances (D-dopa, pyridoxal and salicaldehyde). 3. Compounds were also prepared with an oxazine link between the 3'-oxygen and the nitrogen attached to the 4'-carbon of the aldehyde moiety. 4. None of the D-dopa adducts produced any significant inhibition, but the L-dopa adducts were all active at millimolar levels, with the oxazine derivatives being more active than their parent compounds. 5. Inhibition was competitive with respect to L-dopa, but was neither competitive nor non-competitive with respect to pyridoxal phosphate. 6. The most active compound tested was the oxazine derivative of the L-dopa/salicaldehyde adduct, with an estimated Ki of 58.0 microM. 7. Increased inhibitory activity was observed when enzyme depleted of pyridoxal phosphate was used.

Effects of brocresine (NSD-1055) and cycloheximide on amino acid decarboxylase activities in gastric mucosa of normal and vagally denervated rats

1. Histidine decarboxylase activity of rat stomach fluctuates depending upon the functional state of the stomach. This varying enzyme activity poses special problems in assessing the effectiveness of enzyme inhibitors. After vagal denervation gastric histidine decarboxylase is markedly activated and remains at a high, stable level, which is unaffected by the functional state of the stomach. Thus it appears that vagally denervated rats are well suited for studies on histidine decarboxylase inhibitors.2. In vivo, brocresine (NSD-1055) was found to be a more effective inhibitor of gastric DOPA decarboxylase than of gastric histidine decarboxylase. With the fairly high dose given (200 mg/kg) the inhibition of histidine decarboxylase was at most 75-85% and quite short-lasting. The DOPA decarboxylase activity, which was not affected by vagal denervation, was inhibited more than 95% by brocresine; this inhibition was longer-lasting.3. Cycloheximide, which probably lowers gastric histidine decarboxylase activity by inhibiting enzyme synthesis, was maximally effective at a dose level as low as 1 mg/kg. Gastric DOPA decarboxylase was not inhibited by cycloheximide. Vagotomized rats and control rats responded similarly.4. Combined treatment of vagally denervated rats with brocresine and cycloheximide resulted in a rapid and persistent reduction of the histidine decarboxylase activity. It is concluded that the failure of brocresine alone to induce a lasting inhibition of histidine decarboxylase is due to continuous, rapid synthesis of new enzyme.5. The calculated half-life of gastric histidine decarboxylase was 75 min in vagally denervated rats and 45 min in normal fasted rats. The results suggest that the increased enzyme activity after vagal denervation is caused by an increased rate of enzyme synthesis.

The effect of brocresine (NSD-1055) on the histidine decarboxylase activity in the rat gastric mucosa

The effect of the histidine decarboxylase inhibitor brocresine (NSD-1055) on the specific histidine decarboxylase in the gastric mucosa was investigated in rats. The inhibiting potencies of brocresine were compared after oral and intraperitoneal administration with and without 2-deoxy-d-glucose. Furthermore, different doses of brocresine were added directly to an incubation medium containing an homogenate of the gastric mucosa of untreated animals. The gastric histidine decarboxylase of the brocresine-pretreated animals was not inhibited. Addition of brocresine to the incubation medium produced a dose dependent blockade of the enzyme. 50% inhibition was accomplished by a concentration of 1·4 times 10−6M. The results demonstrated an inhibition of the rat stomach histidine decarboxylase in vitro, but not in vivo, indicating the inability of brocresine to interfere with the biosynthesis of histamine in the rat stomach.