Cationized serum albumin enhances response of cultured fetal rat long bones to parathyroid hormone.

The effects of cationized serum albumin on the resorptive response to PTH and other agents were examined in organ cultures of fetal rat long bones. Human serum albumin (HSA) was cationized to an isoelectric point greater than 9.5 by addition of hexamethylene diamine. When cationized albumin (C-HSA) replaced HSA or BSA in the medium, resorption could be stimulated by 10- to 30-fold lower concentrations of synthetic (1-34) human or bovine PTH or intact 1-84 bovine PTH. Using C-HSA, significant resorption was obtained in some experiments with the concentration of PTH as low as 25 pM, but in most experiments 100 pM-400 pM concentrations were required. In contrast, 6.25 nM 1-34 PTH was required for a response in HSA. The sensitivity to stimulation of resorption by 1,25-dihydroxy-vitamin D and prostaglandin E2 was not increased. Hence, the increased sensitivity to PTH is most likely due to a selective protective effect of C-HSA, which might decrease nonspecific binding or degradation of the hormone.