A B S T R A C T The influence of the serum binding protein (DBP) for vitamin D and its metabolites on the concentration of its main ligands, 25-hydroxyvitamin D3 (25-OHD3) and 1,25-dihydroxyvitamin D3 (1,25-[OHh2D3) was studied. The concentration of both 1,25-(OH)2D3 and DBP in normal female subjects (45±14 ng/liter and 333±58 mg/liter, mean+SD, respectively; n = 58) increased during the intake of estro-progestogens (69±27 ng/liter and 488±90 mg/liter, respectively; n = 29), whereas the 25-OHD3 concentration remained unchanged. A positive correlation was found between the concentrations of 1,25-(OH)2D3 and DBP in these women.At the end of pregnancy, the total concentrations of 1,25-(OH)2D3 (97±26 ng/liter, n = 40) and DBP (616 ±84 mg/liter) are both significantly higher than in nonpregnant females and paired cord serum samples (48±11 ng/liter and 266±41 mg/liter, respectively). A marked seasonal variation of 25-OHD3 was observed in pregnant females and their infants, whereas in the same samples the concentrations of both DBP and 1,25-(OH)2D3 remained constant throughout the year.The free 1,25-(OH)2D3 index, calculated as the molar ratio ofthis steroid and DBP, remains normal in women taking estro-progestogens, however, and this might explain their normal intestinal calcium absorption despite a high total 1,25-(OH)2D3 concentration. In pregnancy the free 1,25-(OH)2D3 index remains normal up to 35 wk of gestation, but during the last weeks of Part of this work was presented at

The concentration of the vitamin D-binding protein was measured in human serum by single radial immunodiffusion. Normal serum concentrations were slightly higher in normal women than in normal men. No race-related difference was found between white people from Belgium and black people from Zaire. Lower concentrations were found in cord serum and in patients with cirrhosis of the liver. Increased serum levels were observed during pregnancy or during the intake of estro-progestogens. The serum level of the vitamin D-binding protein was not altered in various diseases of calcium metabolism (primary osteoporosis, primary and secondary hyperparathyroidism, rickets, osteomalacia or vitamin D intoxication). No correlation was found between serum levels of 25-hydroxy vitamin D and those of its binding protein. From these data the following conclusions can be drawn: 1) The serum concentration of the vitamin D-binding protein (about 6.10(-6)M) largely exceeds the normal serum concentration of 25-hydroxy vitamin D (about 4.10(-8)M), so that this protein is normally for less than 1% saturated, 2) Normal serum levels of the vitamin D-binding protein were observed in several diseases of calcium metabolism, and 3) The free concentration of 25-hydroxyvitamin D is not regulated at a constant level.

The serum concentrations of parathyroid hormone, 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 were measured in patients with untreated thyroid disorders. The serum concentration of parathyroid hormone was decreased in hyperthyroidism [20 +/- 10 mU/liter, (mean +/- SD); n =23; P < 0.01] and increased in hypothyroidism (53 +/- 17 mU/liter; n = 12; P < 0.001) compared to that in normal subjects (26 +/- 9 mU/liter; n = 81). The concentration of 25-hydroxyvitamin D3 was not altered, but the concentration of 1,25-dihydroxyvitamin D3 was significantly lower in the serum of hyperthyroid patients (28 +/- 11 ng/liter) than in the serum of normal subjects (42 +/- 13 ng/liter). On the contrary, an increased concentration of 1,25-dihydroxyvitamin D3 was observed in the serum of hypothyroid patients (73 +/- 28 ng/liter; P < 0.001 vs. normal subjects). The abnormal serum concentrations of 1,25-dihydroxyvitamin D3 in thyroid disorders cannot be explained by differences in serum binding because the serum vitamin D-binding protein was unaltered in hyperthyroid subjects and only slightly increased (+17%) in hypothyroid subjects. These changes in the serum concentration of 1,25-dihydroxyvitamin D3 are compatible with previous data on altered intestinal calcium absorption in thyroid disorders.

Rat prostatic cytosol contains a high concentration of a prostatic binding protein with peculiar steroid-binding properties. Indeed, in spite of a relatively low affinity, charcoal adsorption can be used for its measurement. Furthermore, the binding is not specific for particular steroids and increases very strongly after delipidation. In delipidated cytosol the concentration of the binding site is 3.1 pmolig protein and the apparent affinity for pregnenolone 1.7 x lo6 M-l. The high concentration of prostatic binding protein in prostatic fluid shows that this substance is secreted by the prostate.Prostatic binding protein has the following physicochemical characteristics : it is precipitated by ammonium sulfate between 50 and 70 % saturation; the elution position from a Sephadex G-100 column corresponds to a molecular weight of 51000; it sediments in sucrose density gradients at 3.7 S and is eluted from DEAE-cellulose columns at about 0.25 M KCl. On polyacrylamide gel electrophoresis the binding activity coincides with the major cytosolic protein band. This band has the same mobility as serum albumin in 7 gels, but a higher mobility in more concentrated gels.The last few years there has been strong interest in androgen-receptor binding in rat prostate, a target tissue for these hormones [1-31. This binding is characterized by a low concentration and a high affinity and is thus saturated at low concentrations of steroid. Furthermore, this binding is specific for a limited number of steroids. During these studies on androgen receptors another type of steroid binding was encountered in rat prostate [4 -61. This non-specific and not readily saturable binding has not been studied in detail. In the present paper we have tried to characterize this non-receptor binding in rat prostate. We will present evidence that it is due to a prostatic binding protein [7], which is peculiar to this organ and secreted by the latter. EXPERIMENTAL PROCEDURE MaterialsThe following 3H-labelled steroids were obtained from New England Nuclear (Boston, U.S.A.): 5a-dihydr0[1,2-~H]testosterone (40 Ci/mmol), [1,2-3H]-