Activation of rat brain tryptophan hydroxylase by polyelectrolytes

Abstract

The in vitro activity of rat brain tryptophan hydroxylase is increased 2-fold by heparin and 4-fold by dextran sulfate. Other polysaccharides including hyaluronic acid, chondroitin sulfate and dermatan sulfate, as well as the unsulfated polymer dextran, do not alter hydroxylase activity. The effect of heparin or dextran sulfate on tryptophan hydroxylase is manifested kinetically as a decrease in the apparent K_m of the enzyme for both substrates 6-methyl-tetrahydropterin (6-MPH₄) and tryptophan, as well as increases in the tV_max values. A variety of polyanions (DNA, glycogen, poly-d- and poly-l-glutamic acid) have no effect on tryptophan hydroxylase, whereas salts [NaCl, KCl, (NH₄)₂SO₄ and MgSO₄] inhibit the enzyme, indicating that the effects of heparin and dextran sulfate on tryptophan hydroxylase are not mediated by increases in ionic strength per se. Several lines of data suggest that tryptophan hydroxylase binds ionically to these polyelectrolytes: (1) the activation produced by heparin and dextran sulfate diminishes as the ionic strength of the assay medium increases, (2) tryptophan hydroxylase binds to heparin-substituted Sepharose 4B and is eluted by increasing the ionic strength of the eluant buffer, and (3) large molecular weight dextran sulfate (mol. wt = 500,000) dramatically shifts the elution profile of tryptophan hydroxylase from a K_av of 0.41 to a K_av of 0.10 on a Sepharose/CL-6B column. Taken together, these data suggest that the binding of certain polyelectrolytes to tryptophan hydroxylase may induce a conformational change in the enzyme which results in increased catalytic activity.