(Thyrotropin-releasing hormone releases TSH and PRL; it is not clear whether there is a separate PRF. PIF may be dopamine or some other hormone.)

pGlu*-His-Pro-NH2** GnRH

(Gonadotropic releasing hormone or LHRH; releases LH and FSH, often from the same cell; residue 3, Trp, is important in association with the receptor.) 1 3 10

(Somatotropin release-inhibiting factor or somatostatin; inhibits release of GH and affects release of pancreatic and other hormones.) 1 14

stimulation, are ACTH and /3-lipotropin (and some breakdown of /3-lipotropin to /3-endorphin and y-lipo-tropin), the pars intermedia cells, under the influence of dopamine, produce a-MSH and CLIP (the hydro-lytic products of ACTH) and /3-endorphin. It may be that humans retain these pars intermedia cells, but not in a discrete easily recognized structure.

It can be noted from Figure 3-8 that it is common for releasing hormones to contain unusual constituents at either the N- or C-terminus or at both ends. Pyroglutamate often appears at the N-terminus, and an ami-dated amino acid frequently appears at the C-terminus. These hormones have short half-lives in blood (ca. 3-7 min), and there are enzymes that cleave N- or C-terminal groupings that appear to be essential for their activity. An example of this metabolism applied to TRH is shown in Figure 3-9.

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