Thromboxane B2 uptake and metabolism in isolated perfused lung. Identification and comparison with prostaglandin F2...

Thromboxane B2 was metabolised in isolated perfused guinea-pig lungs to a product identified by negative ion chemical ionisation mass spectrometry as 13,14-dihydro-15-ketothromboxane B2. Conversion was measured by radio TLC and was greater in guinea-pig than rat lungs (29.1 vs. 13.8% at 10 ng/ml), but similar in lungs from normal and sensitized guinea-pigs. Thromboxane B2 metabolism was less than that of prostaglandin F2 alpha but, like it, was prevented at 5 degrees C and reduced by cycloheximide pretreatment. Tissue to medium ratio in perfused guinea-pig lungs was 3.4 for thromboxane B2, but 0.2 for insulin (showing that thromboxane B2 is accumulated within the lung) and was altered after experimental manipulations. Neither lung slices, crude homogenates, cytosolic and microsomal fractions nor purified prostaglandin 15-hydroxydehydrogenase metabolised thromboxane B2 in vitro, although prostaglandin F2 alpha was extensively inactivated. Quantitative partition coefficient and albumin-binding data confirm that thromboxane B2 lacks prominent lipophilicity, implying that cellular uptake in lung must be carrier-mediated. We conclude that thromboxane B2 is a substrate for pulmonary degradation which may form a route for the biological inactivation of thromboxane A2. Its resistance to prostaglandin 15-hydroxydehydrogenase as conventionally tested remains paradoxical and is discussed.