Two fractions with LH releasing activity were obtained from porcine hypothalami by Enzmann et al. (1972). These fractions were designated E 57 and E 203. By comparison with pure natural LH RH and synthetic LH RH, E 203 was identified as purified natural LH RH. E 57, which has only 25% of the activity of E 203, was further evaluated for its biological and immunological activity. E 57 has the same ratio of LH/FSH releasing activity as natural LH RH. The binding to a specific LH RH antiserum is identical with that of natural LH RH and E 203. It is concluded, that E 57 is an artifact of isolation derived from natural LH RH. These findings can be considered as additional evidence for the identity of the hypothalamic LH and FSH releasing hormone, since both gonadotrophins are stimulated by the two hypothalamic fractions.
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