Effect of coenzymes and thyroid hormones on the dual activities of Xenopus cytosolic thyroid‐hormone‐binding protein (xCTBP) with aldehyde dehydrogenase activity

A cytosolic thyroid‐hormone‐binding protein (xCTBP), predominantly responsible for the major binding activity of T 3 in the cytosol of Xenopus liver, has been shown to be identical to aldehyde dehydrogenase class 1 (ALDH1) [Yamauchi, K., Nakajima, J., Hayashi, H., Horiuchi, R. & Tata, J.R. (1999) J. Biol. Chem . 274 , 8460–8469]. Within this paper we surveyed which signaling, and other, compounds affect the thyroid hormone binding activity and aldehyde dehydrogenase activity of recombinant Xenopus ALDH1 (xCTBP/xALDH1) while examining the relationship between these two activities. NAD + and NADH (each 200 µ m ), and two steroids (20 µ m ), inhibit significantly the T 3 ‐binding activity, while NADH and NADPH (each 200 µ m ), and iodothyronines (1 µ m ), inhibit the ALDH activity. Scatchard analysis and kinetic studies of xCTBP/xALDH1 indicate that NAD + and T 3 are noncompetitive inhibitors of thyroid‐hormone‐binding and ALDH activities, respectively. These results indicate the formation of a ternary complex consisting of the protein, NAD + and thyroid hormone. Although the in vitro studies indicate that NAD + and NADH markedly decrease T 3 ‐binding to xCTBP/xALDH1 at ≈ 10 −4   m , a concentration equal to the NAD content in various Xenopus tissues, photoaffinity‐labeling of [ 125 I]T 3 using cultured Xenopus cells demonstrates xCTBP/xALDH1 bound T 3 within living cells. These results raise the possibility that an unknown factor(s) besides NAD + and NADH may modulate the thyroid‐hormone‐binding activity of xCTBP/xALDH1. In comparison, thyroid hormone, at its physiological concentration, would poorly modulate the enzyme activity of xCTBP/xALDH1.