Biochemical Nature of High-Molecular-Weight Prolactin of Human Serum: Identification of a Prolactin-Binding Protein

A. A. Bulatov,^1,2 A. V. Martynov,¹ and N. B. Smirnova¹

¹Endocrinology Research Center, Russian Academy of Medical Sciences, ul. D. Ul'yanova 11, Moscow, 117036 Russia; fax: (7-095) 310-7000.

²To whom correspondence should be addressed.

Submitted April 29, 1997.

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The biochemical nature of a high-molecular-weight immunoreactive prolactin (HMW-irPRL) prepared by gel filtration of women's sera with predominance of this hormone form was studied. Immunochemical characteristics of HMW-irPRL are different from those of 23 kD prolactin (23 kD-PRL). A protein which specifically and reversibly binds to human [¹²⁵I]PRL is isolated from the pooled fractions of HMW-irPRL by affinity chromatography on prolactin-Sepharose. According to gel filtration, the binding protein (BP) has molecular weight about 150 kD, and it reversibly binds to protein A immobilized on Sepharose. Analysis of BP by SDS-PAGE resulted in two major protein bands, of 65-70 and ~150 kD. Both the bands, when transferred to nitrocellulose, interacted with [¹²⁵I]protein A. Binding of highly purified human pituitary prolactin to the BP significantly decreased the immunoreactivity of the hormone. The molecular weight of BP and its interaction with protein A and recognition by poly- and monoclonal antibodies against human (but not guinea pig) IgG indicate that BP may be an immunoglobulin. Thus, our data demonstrate that HMW-irPRL is formed by the binding of 23 kD-PRL to a specific serum protein which is probably an anti-prolactin IgG.

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KEY WORDS: prolactin, high-molecular-weight form, immunoreactivity, human blood serum, binding protein, affinity chromatography, immunoglobulin.

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