c) The Chemical Na ture of the Pitui tary Hormones
Th e pituitary hormon es are proteins or glycopro teins, ranging in mol ecula r we ight
fro m about 45000 down to 1000 (polypeptides). Som e of th e higher values may
result from mol ecular associations, and have been recently revised with th e aid of
improved techniques for their isolation . The amino acid sequence has been determin ed for th e smaller mol ecules including th e neuroh ypophysial hormones, MSH,
corticotrophin as we ll as growth hormone. Considerable differences often exist
in the pre cise amino acid arr an gements in homologou s hormones from various
species, and these somew hat alte r their pharmacolo gical, immunological and ph ysiolog ical propert ies. Such differ ences in activity are especi ally apparent when
equivalent ho rmones from widely divergent sp ecies are cross -tested; thus YVES
FONTAINE (1964) has pointed out that adenohypophysial th yrotrophic gonado tro -
phi c and growth hormon es from fishes, are ineffective when tested on mammals,
though the equi valent hormones of mammals act in th e fishes. Corticorrophins,
which have a smaller molecular weight, can be shown to act in eithe r direction .
Similarl y in the instance of the neurohypophysial hormones, mammalian arginin e
vasopressin (ant idiure tic ho rmone) has an ant idiur etic effect in the frog , Rana esculenta but is 50 times less activ e than the homolo gous amphibian hormone arginine vasotocin from which it only differs by a single amino acid substitution .
(HELLER and BENTLEY, 1965).
a) A denohypophysis. Th e gonadotro phins (FSH and LH) and th e th yrotrophic ho rmon e are glycopro teins wi th mol ecular weights of abou t 30000. Prolactin and
growth hormone are proteins wi th mole cular we ights of about 20000 to 24 000.
Th e amino acid seque nce of human growth ho rmone has been elucidated (L I, LIU,
and D IXON, 1966) and it consists of 54 amino acids . Cort icotrophin is a smaller
molecule (M. W. about 4000) and the sequenc e of its 39 amino acids has been described (and sho wn to differ) in several mammalian species (see EVANS, SPARKS,
and DI XON, 1966). Th e pr ecise structures of melano cyt e-stimulating ho rmones
(MSH) from several mamm als have also been described. In pigs there are two
such active pol yp eptides, P-MSH with 18 to 22 amino acids and a-MSH with 13.
C orticotrophin and MSH exhibit a cross-ove r in th eir biolo gical activities ; MSH
has some corticotrophic actions (onl y 1.5% as strong), whil e corticotrophin can
expand amphibian melanophores. Before th eir structures were known, some confusion existed as to whether th ey really occurred in nature as distinct entities (see
WARING 1963). Knowledge of the amino acid sequences of each shows that th e
13 amino acids of a-MSH are identical with th e N-terminal tridecapeptide portion
of corti cotrophin (see I. H ARRIS, 1966), so that th e crossove r in biological activity
reflects similarities of their structures. As we shall see, other examples of analo gous
biological activity occur betw een different hormones, and th ese should be con sidered when assessing th e ph ysiolo gical significance of a particular hormone in a
species. In addition th e occurrence of such catho lic endoc rine actions may be relevant to assessing the evolution of the ph ysiological role of different hormon es; a
conte mporary ph arm acological action could reflect a past ph ysiological rol e.
Hi sto chem ical techniques have show n the presence of six different typ es of cells
in th e pars distalis, and these have each been assigned to on e of the hormones
53
Th e pituitary hormon es are proteins or glycopro teins, ranging in mol ecula r we ight
fro m about 45000 down to 1000 (polypeptides). Som e of th e higher values may
result from mol ecular associations, and have been recently revised with th e aid of
improved techniques for their isolation . The amino acid sequence has been determin ed for th e smaller mol ecules including th e neuroh ypophysial hormones, MSH,
corticotrophin as we ll as growth hormone. Considerable differences often exist
in the pre cise amino acid arr an gements in homologou s hormones from various
species, and these somew hat alte r their pharmacolo gical, immunological and ph ysiolog ical propert ies. Such differ ences in activity are especi ally apparent when
equivalent ho rmones from widely divergent sp ecies are cross -tested; thus YVES
FONTAINE (1964) has pointed out that adenohypophysial th yrotrophic gonado tro -
phi c and growth hormon es from fishes, are ineffective when tested on mammals,
though the equi valent hormones of mammals act in th e fishes. Corticorrophins,
which have a smaller molecular weight, can be shown to act in eithe r direction .
Similarl y in the instance of the neurohypophysial hormones, mammalian arginin e
vasopressin (ant idiure tic ho rmone) has an ant idiur etic effect in the frog , Rana esculenta but is 50 times less activ e than the homolo gous amphibian hormone arginine vasotocin from which it only differs by a single amino acid substitution .
(HELLER and BENTLEY, 1965).
a) A denohypophysis. Th e gonadotro phins (FSH and LH) and th e th yrotrophic ho rmon e are glycopro teins wi th mol ecular weights of abou t 30000. Prolactin and
growth hormone are proteins wi th mole cular we ights of about 20000 to 24 000.
Th e amino acid seque nce of human growth ho rmone has been elucidated (L I, LIU,
and D IXON, 1966) and it consists of 54 amino acids . Cort icotrophin is a smaller
molecule (M. W. about 4000) and the sequenc e of its 39 amino acids has been described (and sho wn to differ) in several mammalian species (see EVANS, SPARKS,
and DI XON, 1966). Th e pr ecise structures of melano cyt e-stimulating ho rmones
(MSH) from several mamm als have also been described. In pigs there are two
such active pol yp eptides, P-MSH with 18 to 22 amino acids and a-MSH with 13.
C orticotrophin and MSH exhibit a cross-ove r in th eir biolo gical activities ; MSH
has some corticotrophic actions (onl y 1.5% as strong), whil e corticotrophin can
expand amphibian melanophores. Before th eir structures were known, some confusion existed as to whether th ey really occurred in nature as distinct entities (see
WARING 1963). Knowledge of the amino acid sequences of each shows that th e
13 amino acids of a-MSH are identical with th e N-terminal tridecapeptide portion
of corti cotrophin (see I. H ARRIS, 1966), so that th e crossove r in biological activity
reflects similarities of their structures. As we shall see, other examples of analo gous
biological activity occur betw een different hormones, and th ese should be con sidered when assessing th e ph ysiolo gical significance of a particular hormone in a
species. In addition th e occurrence of such catho lic endoc rine actions may be relevant to assessing the evolution of the ph ysiological role of different hormon es; a
conte mporary ph arm acological action could reflect a past ph ysiological rol e.
Hi sto chem ical techniques have show n the presence of six different typ es of cells
in th e pars distalis, and these have each been assigned to on e of the hormones
53