Response of plasma histaminase activity to heparin in normal subjects and in patients with small cell carcinoma of the lung.

Plasma histaminase (PH) activity only partially reflected the consistently high activity of histaminase in tumor tissue of patients with small cell carcinoma of the lung (SCC). To determine whether heparin might release histaminase from tumor tissue into the circulation as it does from some normal tissues, we studied the response of PH activity to small doses of heparin in 41 patients with SCC and in 57 normal subjects. In patients with SCC, the initial mean PH activity (2.0 U/m) was not different from that in normal subjects (1.7 U/ml). After heparin administration, the mean PH activity was no different between the 2 groups (patients with SCC, 3.1 U/ml; normal subjects, 3.5 U/ml). In both groups, the higher the initial PH activity, the greater the magnitude of the PH activity response to heparin. This relationship was linear (normal subjects, r = 0.88; patients with SCC, r = 0.91). However, in patients with SCC, the increase in post-heparin PH activity, as related to the basal levels, was significantly less than that in normal subjects (P less than 0.001 by analysis of covariance). The data suggested that heparin did not affect the release of histaminase from tumor tissue into plasma to the same degree as it promoted the enzyme's entry from normal tissue sources. The differences between the groups may enable us to differentiate between selected patients with SCC and normal subjects with borderline high PH activity values.