The primary sequence of the prolactin (PRL) molecule is shown in Figure 5-15. PRL consists of 199 amino acid residues, and the approximate molecular weight of the glycosylated form is about 25,000. Cleaved forms exist and may have physiological significance or may be simply degradation products.

PRL is a member of a gene family that includes GH and CS (chorionic somatomammotropin). A new member of this gene family has been detected in fish, and this form, "somatolactin," is 24% identical to PRL and GH and represents the largest gene (16 kb) family member. Because of this, somatolactin may be the di-

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