JOURNAL OF CHEMICAL EDUCATION
RECENT RESEARCH ON THYROID HORMONE1 NORMAN KHARASCH University of Southern California, Los Angeles, California
T E A T the iodine-containing secretions of the thyroid iodotyrosine was soon afterward found in thyroid tissue gland exert a profound influence on total body metabo- and its probable role as the precursor of thyroxine suglism has long been known; hut, while much information gested (14). I n 1948 mono-iodotyrosine was detected is now available about hyperthyroidism (Graves' disease) and hypothyroidism (cretinism, myxedema), the major questions about the biochemical role of thyroid hormone are as yet unanswered. The answers cannot be simple, for thyroid hormone is concerned Thyroxine (I) with several aspects of metabolic activity. Salter (1) and others (2-11) have recently written extensive reviews on the subject. As Salter has emphasized, thyroid hormone is not a single, simple chemical entity. Undoubtedly, however, the iodine-containing components of it, in association with thyroglohnlin, are the key substances involved. Crystalline thyroxine (I) was first isolated by Kendall (12) and synthesized by Harington and Barger (15). The L-form is the hiolonicallv active one. Di.. . .
'Summary of a lecture presented before the symposium on The Relation Between Structure and Biological Activity, at the meeting of the American Association for the Advancement of Science, Pullman, Washington, June 23, 1954. The new work mentioned herein, from the author's laboratory, was supported hy grants from ~ l ill^ i and company and the united states Public Health Service.
as a component of thyroid hormone (15) and just last year Gross and Pitt.Rivers (18) and Roche and his coworkers (1y)-by radioautographic graphic means--demonstrated the presence of;% iodothyronine (111) in the sernm of animals given
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prior injections of radioactive iodide. Compound I11 not interfere with the peripheral actions of thyroid horhas several times the activity of I, both in the rat assay mone, has also been pursued. Some progress toward and in tadpole metamorphosis. finding "thyroxine inhibitors'' (6, 18) has been made, but the former efforts have not been successful. These CURRENT TRENDS studies appear worth pushing on with, however, beWhile there was a lull in thyroid hormone research cause of their practical implications. Associated with in the thirties (related to the increasing interest of en- such efforts are attempts to determine which enzyme docrinologists in the steroid hormones) there has been systems are involved with thyroid hormone activity. remarkable renewal of interest in the past 10 t o 15 (6) Many of the clinical aspects of thyroid hormone years in the biosynthesis and biochemical effects of as well as other studies, are presented by authorities in thyroxine and thyroxine analogues. The literature is their special fields in (8) and (11). Iodine metabolism voluminous (over 3000 publications related to thyroid and its relation t o thyroid function has assumed a in a one-year period during 1950-51) and the develop- prominent role in the clinical approach to various pathoments too numerous even to list here. A few of them logical conditions. may, however, he noted: (7) An interesting field of related research lies in the (1) Many new analogues and some isomers of question as to whether thyroxine or its analogues may thyroxine have been synthesized (9, 10, 18) and their have significant effects in species other than vertebiological activities assayed in amphibia and mammalia. brates. While attempts to reveal an unequivocal In this way, new information has been gained about the effect on invertebrates have failed, the recent work of minimum structural requirements for thyromimetic Wainfan, Rittenberg, and Marx ($8)concerning the activity, hut the problem is by no means solved (see effect of thyroxine on the rates of oxidation of cholescorrelation, below). Thyroxine analogues have become terol or glucose, in the aerobacter, A . Aerogenes, is of more readily accessible by means of an important modi- interest. If thyroxine analogues should also cause this fication of the synthesis of substituted diphenyl ethers effect, a convenient assay method may become availdel-eloped by the Glaxo workers (18). able, and the effects of thyroxine in these supposedly (2) Iodine tracer techniques, using Ila', have per- simpler organisms may be suggestive of t,he mode of mitted progress toward understanding of the bio- action of thyroxine in vertebrates. synthesis of thyroid hormone and of its metabolic fate. (8) Niemann and co-workers (9) suggested a hyThe papers of Chaikoff, Taurog, and their associates pothesis of thyroxine action involving a redox equi(19) and those of Rawson, of J. Robbins, and of Kirk- librium with a quinoid form (IV). The evidence which wood (80, 21), among many others, reflect the direc- this has been based on, namely, the activity of the ortho isomer of thyroxine and the inactivitv of the meta tions of some of these studies. (3) The oxidative condensation of two moles of T T " L-di-iodotyrosine (11) has been improved to the extent I H+ 2e of becoming a feasible route to L-thyroxine. The I I mechanism of this reaction (cf. ." (1)) ,, merits further study from the viewpoint of physical-organic chemistry and in relatiou t o the hiosynthesis of the iodinated isomer (which could not f o ~ ma quinoid form) has an thyronines. Studies of the in vitro iodination of interesting counterpart in the recent work of Bruice tyrosine and thyronine have also been made, but the (24), who prepared a met% analogue, of I, having a nature of the thyroid's "iodine trap" h ~ . snot as yet propionic side chain in place of the alanyl group, and been clarified (cf. ($0, $1)). The possibility that sul- reported that this compound has definite thyromimetic fenyl iodides (RSI) may be involved is suggested for activity in the R a w catesbeiana test. The possibility consideration; the rapid formation of these, from thiol that the activity found in this case is caused by a trace groups of proteins and iodine, may occur, and the of the very active analogue ($61, V (R' = H ; R = possibility exists (cf. ($3))that the sulfenyl iodides could -CH&H,COOH; X, X' = iodine) must, however, he considered. A further test of the Niemann hypothesis be effective iodinating agents. would be the synthesis and complete assay of a methylGoiterogenic substances, such as the thiouracils (4) in place of the and 2-mercaptoimidazole, have been developed, their ene homologue of thyroxine (-CH2mode of action partially resolved, and clinical appli- ether oxygen atom in struct,ure I). Such a compound cations have been made. The stages of the biosyn- would also be of interest as a possible competitive inthesis of thyroid hormone have also been partially re- hibitor of thyroxine. Inspection of the literature cited will reveal other vealed by means of these, since the goiterogens can insignificant trends in thyroid hormone research, mention hibit certain phases of the hiosynthesis. (5) Considerable effort has been expended in the of which has necessarily been omitted above. search for agents which could block the peripheral action of thvroxine without disturhiine the normal A CORRELATIVE HYPOTHESIS pituitary-thyioid relation. The alternative objective, The recent work of Bruice, Winzler, and Kharasch to find a substance which could control secretion of (10, 85-27), coupled with the literature on the activthyroid stimulating hormone (TSH) hut which would ities of diphenyl ethers, such as V, suggested a corre
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JOURNAL OF CHEMICAL EDUCATION
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lative hypothesis of the thyromimetic activities of compounds as V, versus their structures. The main
X,X' = I, Br, C1, F, H, CHZ,NO? R' = H or CHs R = ionizable groups as alanyl, -COOH, -CH=CH-COOH, -NHz, etc.
features of this correlation are: (1) Physical parameters which appear sufficiently dominant to base a correlative hypothesis on are: (a) the group dipole moments of X, X' and -OR1, (b) the hydrogen-bonding abilities of X and X', and (c) the ionizing tendency of R. (2) If R is kept constant (e. g., R = D, ~ a l a n y l ) , R' = H or CH2 and X, X' = halogen only, plots of log per cent activity versus the sum of the resultant group dipoles (f p ) of X, X' and R'O-, in the forward and prime rings, give a reasonably linear relation-both for amphibian and mammalian response. (3) Relation number 2 does not hold if X or X' represents H, CH3, or NOz. However, if graphically obtained (empirical) correction factors, h, are deh i s now used) termined and added to f p (i. e., iff p then analogues having substituents H, CH3, or NO2 in the X, X' positions can be plotted together with those of section 2, above. The values of h ("hydrogen bonding factor") are positive--indicative of a positive effect on activity-if the suhstituent groups bond more strongly than halogen (e. g., if X = NOz); and h i s negative for groups such as H or C H , which cannot form hydrogen bonds. A single value for h suffices for H or CH3 in the 3, 5 positions, and another single value (less negative) can be used for H or CH3 in the 3', 5' positions-suggesting the greater importance of hydrogen bonding in the 3, 5 positions. Thus, electron releasing substitutents in the 3', 5' positions can increase thyromimetic activity (cf. (26)); and compounds having -NOz substituents in the 3, 5 positions can show distinct thyromimetic activity, the unfavorable dipole function being partly compensated by the positive value of h for the nitro group. The greater activity of compounds with R' = H versus R' = CHa is ascribed to the greater electron-releasing ability of the hydroxyl versus the methoxy group. The unusually high activity of tri-iodothyronine (15, 95) is not, however, anticipated on the basis of this correlation. (4) If, in a series of molecules related to V, only R is varied, thyromimetic activity for amphibian response is highest (130 times D, ~4hyroxine)for the analogue having the propionic acid side chain, -CHz-CHrCOOH, and activities are lower for compounds having side chains such that their acidities are either greater or less than the propionic acid analogue. This relation -CH=CHincludes groups as -CH2-CHrCOOH, COOH, -COOH, -NH2 etc., hut not the bifunctional
+
alanyl group. On this basis, it appears that penetration versus ionization are important factors in determining thyromimetic activity of molecules related to V, maximum activity (in amphibian response) being exhibited by that molecule which has the most favorable structure for penetrating in the nonioniaed form and exerting its activity in the ionized condition. The above correlation and discussion indicates a picture of thyroxine analogue action, in which the analogue hinds to an active site (presumably an enzyme surface) through the ionized side chain and, primarily, through the groups in the 3, 5 positions-the binding of the back ring being of lesser importance. Once attached, the function of the thyroxine analogue (whatever this may, indeed, be) appears to he related t o the electron density of the diphenyl-ether nucleus. LITERATURE CITED (> 1, ) SALTER. W. T.. in PINCUS.G.. AND K. V. T ~ M A N EdiN.
tors, he ~brmones,"~cadernicPress, Inc., New ~ o r k , 1950, Vol. 11. ( 2 ) ALBERT, A., Ann. Reu. Physiol., 14, 481 (1952). C. R., "The Thyroid Gland," Oxford Univer( 3 ) HARINGTON, sity Press, Oxford, 1933. J., AND R. MICHEL, Advances in Protein Chem., 6 , ( 4 ) ROCKE, 253 (1951). C. P.. J. Am. P h a m . Assoc., Sci. Ed., 40, 595 (51 . . LEBLOND. (1951): (6) MACLAGAN, N. F., AND J. H. WILKINSON, Ann. Rep. OnPWl. Chem. (Chem. Soc. London), 49, 291 (1953). S . B., Phy8iol,Rev.,31,205 (1951). ( 7 ) BARKER, P., AND S. ZUCKERMAN, Editors, Memmra of the ( 8 ) ECKSTEIN, Society for Endocrinology ( L a d a ) , N o . 1, July, 1953. (9) NIEMANN, C., Forlsehr. Chem. w g . Nalurstoffe, 7 , 167 (1950). T . C., Doctoral Dissertation, University of Southern (10) BRUICE, California, Los Angeles, 1954. K. H., '(Textbook of Endoerin010gy," Saunders (11) WILLIAM, Publishing Co., Philadelphia, 1950, Chap. 3. E. C., J . B i d . Chem., 39,125 (1919);cf. also ibid., (12) KENDALL, 40, 265 (1920), and "Thyroxine," Chem. Catalog CO., New York, 1929. C. R., AND G. BARGER, Biochem. J. (London), (13) HARINGTON, 21,169(1927). HARINGTON, C. R., Endocrinology, 49,401 (1951). FINK.K.. AND R. M. FINK.Seiaee, 108. 358 (1948). GRGS'S, J: R., AND R. PITT-RIVERS; ~ i o c h e mJ. . (London), 53, 645 (1953);ibid., 53, 652 (1953). ROCHE, J., S. LIS~TZKT, AND R. MICKEL, Compt. rend., 234, 1228 (1952). BARNES, J. H., ET AL., J. Chem. Soc., 1953, 1448. A,, W. TONG,AND I. L. CHAIKOFP, J. Biol. Chem., . . TAUROG, 1951,677: R. W., Federation Proc., 13.663 (1954). (20) RAWSON, (211 D. M.. AND S. KIRKWOOD. J. B i d . Chem., 205, \--, ~?AWCETT. 793 (19'53). ' (22) KHARASCH, N., S. J. POTEMPA, AND H. L. WEBRMEISTER, Chem. Revs., 39,269 (1946). E., S . C. RITTENBERG, AND W. MARX,Arch. (23) WAINPAN, Biochem., 51, 519 (1954). T. C., J. Org. Chem., 19, 333 (1954). (24) BRUICE, T. C., N. KRARASCH, AND R. J. WINZLER, ibid., ( 2 5 ) BRUICE. 18,83(1953): Rnrnc~.T. C.. R. J. WINZLER. AND N. KHARASCH, J. B i d . them:, 210, i (1954). BRUICE, T. C., N. Kn*a*scn, AND R. J. WINBLER, Paper presented before the Division of Biochemistry at the 123rd Meeting of the American Chemical Society, Los Angeles, March, 1953.
