Effect of histidine decarboxylase inhibitors and other drugs on histamine formation in vivo

Endogenous histamine in cultured bovine adrenal chromaffin cells

Histamine releases catecholamines and opioids in primary cultured bovine adrenal medullary (BAM) chromaffin cells. We have studied whether histamine is synthesized and localized in BAM cells, and whether it can be released upon activation with secretagogues. In BAM cells histamine is immunohistochemically co-localized with tyrosine hydroxylase in 45 +/- 8% of all cells. Only histamine immunoreactivity was observed in 8 +/- 2% of all BAM cells. No mast-cell-like cells were observed in our system. Histamine can be released from BAM cells by high potassium (56 mM K+) in a calcium-dependent manner. Compound 48/80 did not release histamine from BAM cells but nicotine caused a dose-dependent liberation of the amine. Cultured BAM cells have histidine decarboxylase activity which is inhibited by alpha-fluoromethylhistidine. These results indicate that endogenous histamine is synthesized, stored and released in BAM chromaffin cells in vitro.

In vivo and in vitro inhibition of human histidine decarboxylase by (S)-alpha-fluoromethylhistidine

Histidine decarboxylase (HDC) activity in Ficoll-Hypaque purified human peripheral blood leukocytes (PBL) was determined by measuring the formation of [3H]histamine from L-[3H]histidine. HDC activity was inhibited in vitro to more than 90% by (S)-alpha-fluoromethylhistidine (alpha-FMH) at concentrations of 10(-5) M and above. Both polymorphonuclear and mononuclear cells possessed HDC activity, but on a per cell basis the former had several-fold higher enzyme activity than the latter. In safety and tolerability studies, alpha-FMH was administered orally to healthy human subjects twice daily for 7 days at doses of 2.5, 10, 50 and 100 mg per person. A dose-dependent inhibition of HDC activity was observed in PBL that were isolated both at 12 hr after administration of the first dose of alpha-FMH and after treatment for 1 week. At the 50 and 100 mg doses of alpha-FMH, there was complete inhibition of HDC activity and partial inhibition at the 10 mg dose. Twenty-four hours after the last dose, HDC activity had recovered to 64-100%, 44-46%, and 30-52% of control values in subjects that received 10, 50 and 100 mg alpha-FMH respectively.

Inhibitory effect of tritoqualine (TRQ) on histamine release from mast cells

Tritoqualine (TRQ), used clinically as an antiallergic drug, did not inhibit histidine decarboxylase activity (HDC, EC. 4.1.1.22.) partially purified from fetal rats and the enzymes prepared from mastocytoma P-815 cells. However, TRQ inhibited the histamine release from rat peritoneal mast cells induced by compound 48/80 and ATP. TRQ was also effective in inhibiting antigen-induced histamine release in rat mast cells sensitized actively or passively by the homologous anti-DNP-Ascaris antibody. Preincubation of cultured mastocytoma P-815 cells in a medium including TRQ inhibited non-cytotoxically the histamine release of mastocytoma cells induced by compound 48/80, and the effect of TRQ became more marked with lengthening of the culture period in the presence of TRQ. It was concluded from these results that one of the main actions of TRQ as an antiallergic drug was not the inhibitory action on HDC, but might be ascribed to its inhibitory effect on histamine release from mast cells.

Histamine synthesis in intact and disrupted rat mast cells

Histamine production by purified intact rat peritoneal mast cells, as measured by formation of [beta-3H]histamine from [beta-3H]L-histidine or by release of 14CO2 from 14C-carboxyl-labeled histidine, was ten to thirty times greater than that of disrupted cells of soluble extracts of these cells. Loss of activity was evident whether cells were disrupted by sonification, freezing and thawing, or lysis, both in the absence and presence of inhibitors of proteolytic enzymes and agents known to preserve enzyme responsible for histamine formation in both the intact cells and cell extracts. In the presence of subsaturating concentrations of histidine, various histidine analogs and glutamine inhibited histidine data indicate that, at physiological concentrations of histidine, blockade of histidine transport (through system N) may limit histamine synthesis in the intact cell and that measurement of histidine decarboxylase activity in tissue homogenates or cell extracts may not reflect actual histidine decarboxylase activity in vivo.

Methyl derivatives of histamine; interaction with histamine metabolism

Several histamine analogs, with ring or side chain methyl groups, were tested for interaction with certain aspects of 14C-histamine metabolism. The physiological catabolite, denoted as 3-methylhistamine under the nomenclature we have adopted, is known to inhibit histamine methylation in vitro and in vivo. At low concentrations, all analogs inhibited histamine methylation by mouse brain homogenates; however 3-methylhistamine was completely ineffective against the highly active enzyme from rat didney, while 2-methylhistamine was most effective. None of the drugs showed a significant effect on in vivo formation of 14C-histamine by mouse stomach. All analogs had a definite effect on the distribution and fate of intravenously injected 14C-histamine. The 2-methyl analog was the strongest in vivo inhibitor of histamine methylation, and 3-methylhistamine next. The side chain N-methylated histamies altered uptake of 14C-histamine in some tissues but inhibited methylation weakly, if at all. Data on blood kidney and urine for 4-methylhistamine were unique, and suggested some effect on tubular reabsorption. 2-methylhistamine may prove to be the inhibitor of choice for in vivo studies on histamine methylation.