Histamine synthesis in intact and disrupted rat mast cells

Histamine production by purified intact rat peritoneal mast cells, as measured by formation of β-³H]histamine from β-³H]l-histidine or by release of ¹⁴CO₂ from ¹⁴C-carboxyl-labeled histidine, was ten to thirty times greater than that of disrupted cells or 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 activity. Studies with decarboxylase inhibitors indicated that a specific histidine decarboxylase was 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 uptake and histamine formation in intact mast cells but did not inhibit synthesis in cell extracts. These 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.