Plasma and cerebrospinal fluid (CSF) specimens from 3 patients with suprasellar extension of a pituitary tumour were fractionated by column chromatography. 'Big' and 'little' prolactin were found in plasma but only 'little' prolactin was found in CSF. Prolactin containing CSF from 2 of the patients was incubated with prolactin-free plasma and chromatographed. The elution patterns demonstrated a striking shift toward the [125I]albumin marker resembling the 'big' and 'little' prolactin pattern seen in native plasma. In addition, [125I]h prolactin was incubated with prolactin-free plasma and rechromatographed. A substantial early eluting peak of radioactivity occurred in the zone of albumin elution. The presence of only 'little' prolactin in CSF and the conversion of CSF 'little' prolactin to 'big' prolactin by exposure to plasma suggest that 'big' prolactin is not a prohormone but results from binding of 'little' prolactin to plasma proteins or aggregation of 'little' prolactin in the presence of plasma.
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