Pyridoxal phosphate: A possible cofactor in steroid hormone action

Pyridoxal phosphate has been shown to interact with cytoplasmic glucocorticoid receptors and alter their molecular properties. We have now used pyridoxal phosphate to extract and characterize nuclear receptors from [ 3H]-dexamethasone treated isolated rat thymocytes. Pyridoxal phosphate effectively extracts glucocorticoid receptors from isolated thymocyte nuclei in a time dependent and dose dependent manner. Concentrations of the vitamin as low as 3 × 10 −4M extract 50% of nuclear [ 3H]-dexamethasone receptor complexes within 30 min at 0°C. Furthermore, mM concentrations of pyridoxal phosphate were as effective as 0.4 M KCl in extracting nuclear receptors. In addition, pyridoxal phosphate extracted NaBH 4 reduced receptors were more stable than KCl extracted receptors. The extractability of the steroid receptor complexes is related to the biological potency of the ligand used to interact with receptor. For example, cortisol-receptor complexes are extracted more efficiently than either dexamethasone or ti-iamcinolone acetonide receptor complexes. Dexamethasone receptors extracted from nuclei by pyridoxal phosphate and reduced with NaBH 4 were found to be identical in size and shape (sedimentation coefficient 2.9 S, Stokes radius 42 Å) to cytoplasmic receptors (unactivated or activated) treated with the vitamin and NaBH 4. In addition, the glucocorticoid receptor extracted by pyridoxal phosphate but not 0.4 M KCl became insensitive to trypsin digestion. These studies show that vitamin B-6 can extract intact nuclear glucocorticoid receptors at near physiological concentrations and protect these receptors from proteolytic degradation. We speculate that pyridoxal phosphate may be involved in the cytoplasmic recycling and perhaps reutilization of glucocorticoid receptors.