Histidine decarboxylase from rat and rabbit brain: partial purification and characterization.

Histidine decarboxylase, the synthetic enzyme for histamine, was partially purified from regions of rat or rabbit brain rich in the enzyme. The enzyme was purified using ion exchange and hydrophobic column chromatography and chromatofocusing. Approximately 70-fold and 110-fold enrichments were attained from rat and rabbit brain, respectively. Rat and rabbit brain histidine decarboxylase had isoelectric points of pH 5.4 and 5.6, Km values of 80 microM and 120 microM histidine and Vmax values of 210 and 625 pmol histamine formed/hr-mg protein, respectively. The partially purified histidine decarboxylase from both sources was dependent on pyridoxal phosphate for maximal activity and was inhibited by alpha-fluoromethylhistidine, nickel chloride and cobaltous chloride but was not inhibited by impromidine, alpha-methyldopa, DTNB, zinc chloride or mercuric chloride. The enzyme had a broad pH optimum between pH 7.2 and 8.0. These studies provide further information on the characteristics of mammalian histidine decarboxylase from brain.

Histamine synthesis in rat hypothalamus is not acutely regulated via histidine decarboxylase.

There is considerable support for histamine acting as a neurotransmitter in the central nervous system. Since the synthesis of many biogenic amine neurotransmitters is highly regulated on a short-term basis, we investigated whether a similar regulation exists for histamine synthesis. Previous studies have indicated that histidine transport into nerve terminals is not a regulatory step in histamine synthesis. In this study we examined histidine decarboxylase activity as a possible regulatory point. Using a variety of depolarization methods and hypothalamic and striatal tissue slices and/or synaptosomes, we found no alterations in histidine decarboxylase activity. Based on these and previous studies we conclude that histamine synthesis in the hypothalamus, and perhaps throughout the CNS, does not have mechanisms for rapid short-term regulation. These data support the idea that histamine acts as a neuromodulator and does not play a critical role in acute regulation of CNS function at least in some brain regions.

Transport of histidine into synaptosomes of the rat central nervous system.

Histidine transport into synaptosomes was studied in order to characterize this aspect of histamine synthesis in neurons. Histidine transport was found to be independent of sodium, calcium, and magnesium ions and dependent upon potassium and chloride ions. Histidine transport was also found to be energy dependent, and fractionation studies suggested it was highly localized to nerve terminals. Kinetic analysis of histidine transport in several brain regions indicated the presence of two uptake sites, a high-affinity site with a Km of approximately 35 microM and a low-affinity site with a Km in the millimolar range. Density of the high-affinity site, as reflected by Vmax, correlates well with density of proposed histaminergic innervation. Rate of histidine transport was not altered by prior depolarization of the synaptosomes, indicating that histidine transport probably does not play a regulatory role in histamine synthesis.