Absence of the plasma growth hormone-binding protein in Laron-type dwarfism

We recently described a specific, high affinity binding protein (BP) for growth hormone (GH) in normal human plasma. Little is known about the source, regulation and biological role of this BP. Because its specificity is similar to that of the GH receptor, we considered the possibility that it represented a circulating receptor subunit or fragment. Laron-type dwarfism is a rare disorder characterized by severe growth failure, resistance to GH, and GH receptor deficiency. To probe the relationship between the receptor and the circulating binding protein, we measured the binding of GH to plasma from a child with Laron-type dwarfism and compared it with that in plasma of normal children and adults. Normal plasma samples were supplemented with unlabeled GH to the endogenous GH level in the plasma of the Laron patient to yield comparable saturation of the BP. After incubation of plasma with radiolabeled GH, bound GH was separated from free GH by gel filtration on Sephadex G-100. There was no detectable binding (less than 1.5%) of GH in the plasma of the Laron-type dwarf, whereas in normal children the bound GH fraction averaged 24.0 +/- 6.1% (mean +/- SD). Thus, the GH-BP is either absent or functionally abnormal in Laron-type dwarfism. This finding suggests that the circulating GH-BP is related to and/or may be derived from the GH receptor. Alternatively, it is possible that the BP plays an as yet poorly understood pivotal role in the promotion of somatic growth.