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 T3 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 microm), and two steroids (20 microm), inhibit significantly the T3-binding activity, while NADH and NADPH (each 200 microm), and iodothyronines (1 microm), inhibit the ALDH activity. Scatchard analysis and kinetic studies of xCTBP/xALDH1 indicate that NAD+ and T3 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 T3-binding to xCTBP/xALDH1 at approximately 10-4 m, a concentration equal to the NAD content in various Xenopus tissues, photoaffinity-labeling of [125I]T3 using cultured Xenopus cells demonstrates xCTBP/xALDH1 bound T3 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.
Yamauchi
Kiyoshi
K
Department of Biology and Geoscience, Faculty of Science, Shizuoka University, Shizuoka, Japan. sbkyama@ipc.shizuoka.ac.jp
Nakajima
Jun-ichiro
J
D000444:Aldehyde Dehydrogenase / Q000037:antagonists & inhibitors / Q000502:physiology*; D000818:Animals; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000378:metabolism; D004926:Escherichia coli; D007700:Kinetics; D008565:Membrane Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D009243:NAD / Q000502:physiology; D011485:Protein Binding / Q000502:physiology; D011994:Recombinant Proteins / Q000378:metabolism; D013963:Thyroid Hormones; D014284:Triiodothyronine / Q000502:physiology*; D014981:Xenopus; D000097810:Thyroid Hormone-Binding Proteins
D000444:Aldehyde Dehydrogenase; D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D008565:Membrane Proteins; D009243:NAD; D011994:Recombinant Proteins; D013963:Thyroid Hormones; D014284:Triiodothyronine; D000097810:Thyroid Hormone-Binding Proteins