5'-deoxypyridoxal interaction with dexamethasone receptor: a new probe for structure and function of steroid receptors.

5'-Deoxypyridoxal, a vitamin B-6 analogue, increased the rate of dissociation of [3H]dexamethasone from HeLa S3 cytoplasmic glucocorticoid receptor complexes in vitro. This effect was achieved at millimolar concentrations of 5'-deoxypyridoxal, suggesting a low-affinity interaction of 5'-deoxypyridoxal with receptor. Loss of [3H]dexamethasone-receptor binding in the presence of 5'-deoxypyridoxal was pH dependent, and a plot of Kdiss vs. pH fit a simple sigmoidal titration curve with an inflection point at pH 7.8, suggesting that deprotonation of a single functional group on 5'-deoxypyridoxal increases Kdiss. Loss of [3H]dexamethasone binding in the presence or absence of unlabeled steroid also increased with pH, but no inflection point occurred over the range of pH tested. A titration of 5'-deoxypyridoxal indicated a pK of 7.94 for the pyridinium proton, suggesting deprotonation of the pyridinium nitrogen may account for the pH dependence of Kdiss of dexamethasone from receptor. 5'-Deoxypyridoxal also caused a decrease in nuclear [3H]-dexamethasone-receptor binding when incubated with whole HeLa S3 cells at 37 degrees C. Furthermore, 5'-deoxypyridoxal was effective in reducing nuclear binding of dexamethasone when added either simultaneously with [3H]dexamethasone or after achievement of equilibrium of steroid with receptor. The reduction in nuclear [3H]dexamethasone binding is highly specific for 5'-deoxypyridoxal. Several analogues of this compound, including 5'-deoxypyridoxamine, were ineffective. In addition, this effect was reversible following removal of extracellular 5'-deoxypyridoxal. Under these conditions, 5'-deoxypyridoxal was competitive with dexamethasone for binding to nuclear receptor, with KI = 8.1 X 10(-6) M. Scatchard plot analysis of dexamethasone-receptor binding in the presence or absence of 5'-deoxypyridoxal was consistent with an apparent reduced affinity of [3H]dexamethasone for receptor, which again suggests competitive interaction or allosteric interaction mediated dissociation. Glucocorticoids are known to stimulate alkaline phosphatase activity within HeLa S3 cells. In whole cell incubations, 5'-deoxypyridoxal was effective in reducing the dexamethasone-induced increase in alkaline phosphatase activity by 60% under conditions in which cell viability and cell growth were not affected.