HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Kahl, S. Articles by Rumsey, T. S. Search for Related Content PubMed PubMed Citation Articles by Kahl, S. Articles by Rumsey, T. S.

In Vitro Studies of the Effect of Propylthiouracil and Ronnel on Thyroxine-5'Monodeiodinase Activity in Steers¹

University of Maryland, College Park 20742 and U.S. Department of Agriculture, Beltsville, MD 20705

Abstract

This study compares in vitro effects of propylthiouracil (PTU) and ronnel on the conversion of thyroxine (T₄) to triiodothyronine (T₃) in liver and kidney from Angus steers. Tissues were homogenized and incubated with T₄ (1.3 µM) in the presence of 0 to 59 µM PTU or 0 to 49.7 µM ronnel. The T₃ generated during a 30-min incubation was measured by radioimmunoassay. It was found that 1.47, 5.9 and 59 µM PTU decreased T₄ to T₃ conversion in liver and kidney by 62 and 88, 71 and 100, and 81 and 100%, respectively. The inhibition caused by 1.47 and 5.9 µM PTU was overcome by addition of 2 mM dithiothreitol (DTT). Ronnel in concentrations of 1.24, 6.22, 49.7 µM decreased T₄ to T₃ conversion in liver and kidney 46 and 45, 51 and 72, and 78 and 95%, respectively. However, with ronnel, the addition of DTT caused further inhibition. A Lineweaver-Burk plot of the data obtained using .32 to 6.43 µM T₄ with 1.47 and 5.9 µM PTU or 6.22 and 12.44 µM ronnel indicated that PTU is an uncompetitive inhibitor (K_i = 1.67 µM) and ronnel is a noncompetitive (K_i = 15.5 µM) inhibitor of T₄-5'-monodeiodination. The data suggest that decreased conversion of T₄ to T₃ by PTU or ronnel may be responsible for the increased plasma concentrations of T₄ and slightly decreased plasma concentrations of T₃ reported in steers treated with levels of both PTU and ronnel that are associated with growth stimulation.

¹ Scientific article No. A-4047, contribution No. 7032 of the Maryland Agr. Exp. Sta.

² Visiting Scientist from Academy of Agr., Krakow, Poland at Univ. of Maryland, Dept. of Anim. Sci., College Park 20742.

³ Milk Secretion and Mastitis Lab., Anim. Sci. Institute. To whom reprint requests and correspondence should be addressed.

⁴ Ruminant Nutrition Lab., Anim. Sci. Institute.