Interaction of cytoplasmic tyrosine hydroxylase with chromaffin granule. In vitro studies on association of soluble enzyme with granule membranes and alteration in enzyme activity.

The interaction of soluble tyrosine hydroxylase with isolated chromaffin granule membranes was studied. The incubation of the granule membranes with the soluble fraction in the mixture that may approximate the intracellular environment of the resting cell resulted in a marked increase in the enzyme activity recovered in the membrane fraction, and enzyme activity precipitated with the granule membranes was increased according to incubation time and dependent on amounts of both the granule membranes and soluble fraction. The association of the soluble enzyme with the granule membranes was reversible and specific, whereas a decrease in activity of the soluble enzyme was observed when the soluble fraction was incubated with the granule membranes in the mixture in which association of the soluble enzyme with the granule membranes might occur. These results seem to indicate that activity of the soluble enzyme is presumably modulated by the granule through association of the soluble enzyme with the surface of the granule, thus suggesting that interaction between tyrosine hydroxylase and the chromaffin granule may play a possible role in regulation of catecholamine biosynthesis as a consequence of modulating activity of the rate-limiting enzyme within the cell.