Release of vitamin B12--binding protein by human leukocytes in vitro

Human granulocytes (G) contain a vitamin B(12)-binding protein (B(12)BP). There is evidence that chronic myelogenous leukemia leukocytes (CMLL) may synthesize B(12)BP. Our prior studies suggested that intact, living intravascular G synthesize and release such protein into extracellular compartments in vivo. In the present study, CMLL were incubated in Trisbuffered Hank's basal salt solution (pH 7.2) containing 0.1% human serum albumin to study release of B(12)BP into the medium. B(12)BP was released continuously and in increasing amounts over a 5 hr period at 37 degrees C; this release was inhibited almost completely when the cells were incubated at 4 degrees C and by about half as much in the presence of N-ethylmaleimide (1 mmole/liter). Cycloheximide (50 mug/ml) had no effect on the release of B(12)BP but significantly inhibited incorporation of leucine-(3)H into leukocyte protein. G incubated with 20 mg/ml of compound 48/80, an experimental histamine-releasing agent, had a 6-fold increase in release of B(12)BP over a 2 hr period. Subcellular fractionation studies of human granulocytes demonstrate that most of the B(12)BP is associated with the granular (20,000 g) layer with an excellent correlation observed between its subcellular distribution and that of acid phosphatase.These findings suggest that the release of B(12)BP from G is mediated by an active process and provide further evidence that granulocytes are secretory as well as phagocytic cells.