Inhibition of Catalase Activity Mediates the Actions Of 24,25(OH)2D3

Incubation of enterocytes with 6.5 nM 24,25(OH) 2 D 3 decreased catalase activity and increased H 2 O 2 production in a parallel manner. An antibody against a putative 24,25(OH) 2 D 3 binding protein was found to neutralize the inhibitory effect of the steroid on 1,25(OH) 2 D 3 ‐mediated 32P uptake (P<0.05 to 0.001 at T=3–10 min of incubation). Incubation of cells in the presence of 50 nM catalase was also found to alleviate inhibition at T=5–10 min of incubation. In another series of experiments, isolated intestinal epithelial cells were incubated as controls or with 1,25(OH) 2 D 3 , each in the presence of the catalase inhibitor 3‐amino‐1,2,4‐triazole, or with 1,25(OH) 2 D 3 alone. Cells exposed to hormone alone again showed an increased accumulation of 32 P from T=5–10 min, while cells treated with catalase inhibitor and hormone had uptake levels that were indistinguishable from controls. We tested whether inactivation of protein kinase C (PKC), the signaling pathway for 32 P uptake, occurred. Incubation of cells with 100 nM phorbol‐13‐myristate (PMA) increased 32 P uptake to 144% of controls, while cells pretreated with 50 μM H 2 O 2 prior to PMA did not exhibit increased uptake. Likewise, PMA significantly increased PKC activity at T=1–3 min (P< 0.05, relative to corresponding controls), while cells exposed to H 2 O 2 prior to PMA did not. We conclude that catalase has a central role in mediating rapid responses to steroid hormones.