1 alpha-hydroxy-25-fluorovitamin D3: a potent analogue of 1 alpha,25-dihydroxyvitamin D3

Chemically synthesized 1 alpha-hydroxy-25-fluorovitamin D3 was compared to 1,25-dihydroxyvitamin D3 for potency in the chick intestinal cytosol-binding protein assay, induction of intestinal calcium transport, mobilization of calcium from bone, and epiphyseal plate calcification in the rat. The 25-fluorinated analogue causes 50% displacement of 1,25-dihydroxy[23,24-3H]D3 at 1.8 X 10(-8) M in the competitive protein-binding assay, whereas only 5.6 X 10(-11) M of unlabeled 1,25-dihydroxyvitamin D3 is needed for equal competition. This 315-fold difference between and 1 alpha-hydroxy-25-fluorovitamin D3 indicates that the fluoro analogue is about equipotent with 1 alpha-hydroxyvitamin D3 in the protein-binding assay. However, 1 alpha-hydroxy-25-fluorovitamin D3 is 1/50 as active as 1,25-dihydroxyvitamin D3 in vivo in the stimulation of intestinal calcium transport and bone calcium mobilization in vitamin D deficient rats on a low-calcium diet. Likewise, 1 alpha-hydroxy-25-fluorovitamin D3 is about 40 times less active than 1,25-dihydroxyvitamin D3 in inducing endochondrial calcification in rachitic rats. No selective actions of 1alpha-hydroxy-25-fluorovitamin D3 were noted. Since the 25 position of the analogue is blocked by a fluorine atom, it appears that 25-hydroxylation of 1 alpha-hydroxylated vitamin D compounds in vivo is not an obligatory requirement for appreciable vitamin D activity.     

The influence of dietary calcium and phosphorus on intestinal calcium transport in rats given vitamin D metabolites.

A new method for the simple analysis of methylated amino acids based on autoradiography is introduced. With this technique a survey of protein methylation in a prokaryote, Escherichia coli, and a eukaryote, fibroblasts in culture, was carried out in an attempt to identify, quantitate, and determine the subcellular localization of all the methylated amino acids found in the proteins of these organisms.In mammalian cells using an established mouse fibroblast line (3T3), we have found that nuclei-free and mitochondria-free cytoplasm contain readily detectable amounts of four identifiable methylated amino acids: N^?,N^?-dimethyllysine, N^?,N^?,N^?-trimethyllysine, N^G,N^G-dimethylarginine (or N^G-methylarginine), and N^G,N′^G-dimethylarginine. The crude nuclear pellet also contains these methylated amino acids, but in addition contains N^?-methyllysine and a new as yet unidentified methylated compound. Histones purified from these nuclei contain essentially the same array of methylated compounds.The ribosomal subunits of the mammalian cells contained only small amounts of the methylated amino acids; the 40S subunit contained a substantial amount of just one, N^G,N^G-dimethylarginine (or N^G-methylarginine), and smaller amounts of N^G,N′^G-dimethylarginine, and an as yet unidentified methylated compound. The 60S subunit contained even smaller amounts of methylated amino acids, 50% of which was N^?,N^?,N^?-trimethyllysine and smaller amounts of N^?-methyllysine, N^?,N^?-dimethyllysine, and N^G,N^G-dimethylarginine. These subunits also contained an as yet unidentified methylated compoundThese results were in marked contrast to those that we obtained with the prokaryote, Escherichia coli. Only the proteins of the 50S ribosomal subunit of the bacteria contained methylated amino acids. Of those present 50% was N^?,N^?,N^?-trimethyllysine, with the remainder distributed about equally between N^?-methyllysine and three unknowns, one of which is apparently the same as that found in the 60S subunit of the mouse fibroblasts. All of the N^?-methyllysine was apparently in the small acidic proteins, L7 and L12.     

The role of 1,25-dihydroxyvitamin D3 and parathyroid hormone in the regulation of chick renal 25-hydroxyvitamin D3-24-hydroxylase.

A single 325-pmol dose of 1,25-dihydroxyvitamin D₃ given to chicks fed a vitamin D-deficient diet containing 3% calcium and 0.6% phosphorus suppresses renal mitochondrial 25-hydroxyvitamin D₃-1α-hydroxylase and stimulates the 25-hydroxyvitamin D₃-24-hydroxylase as measured by in vitro assay. This alteration in the enzymatic activity takes place over a period of hours. The administration of parathyroid hormone rapidly suppresses the 25-hydroxyvitamin D₃-24-hydroxylase. The alterations in the hydroxylases by parathyroid hormone or 1,25-dihydroxyvitamin D₃ are not related to changes in serum clacium or phosphate but could be related to changes in intracellular levels of these ions. Actinomycin D or cycloheximide given in vivo reduces the 25-hydroxyvitamin D₃-24-hydroxylase activity rapidly which suggests that the turnover of the enzyme and its messenger RNA is rapid (1- and 5-h half-life, respectively). The half-lives of the hydroxylases are sufficiently short to permit a consideration that the regulation by 1,25-dihydroxyvitamin D₃ and parathyroid hormone may involve enzyme synthesis and degradation.     

An in vitro study of vitamin D2 hydroxylases in the chick.

An in vitro study of the liver 25-hydroxylation of vitamin D2 and the kidney 1- and 24-hydroxylations of 25-hydroxyvitamin D2 was undertaken in order to determine whether the discrimination against vitamin D2 seen in chicks in vivo is the result of a block of one or more of the steps in the activation of the vitamin D2 molecule. Vitamin D2 hydroxylation reactions in the chick are virtually identical with those observed with the vitamin D3 series. It is, therefore, concluded that the chick possesses the required enzymatic machinery to synthesize 1,25-dihydroxyvitamin D2 and that the discrimination must be because of some unknown metabolic reaction of the vitamin D2 compounds, to a defect in the transport of vitamin D2 metabolites, or to target organ discrimination.     

Multiple forms of vitamin D-dependent calcium-binding protein in rat kidney

The technique of two-dimensional electrophoresis was used in combination with a highly sensitive silver stain to study vitamin D-dependent calcium-binding protein (CaBP) in rat kidney. Rat renal CaBP was shown to co-migrate almost exactly with CaBP purified from chick intestine suggesting evolutionary conservation of this protein. In some cases rat renal CaBP appeared not as a single polypeptide, but rather as a cluster of 4 polypeptides. Formation of the satellite cluster of CaBP in response to high doses of 1,25-dihydroxyvitamin D3 occurred in young rats which had been maintained on a vitamin D-deficient diet for 2 weeks, as well as in older rats which had been maintained on the same diet for 5 months. The 4 forms of CaBP were not the result of various states of Ca2+ binding, but rather the result of an enzymatic reaction. This was shown by 3 experiments. 1) Adding excess EGTA to samples containing the 4 satellite forms did not change the two-dimensional electrophoretogram. 2) Incubation of purified chick intestinal CaBP with kidney cytosols from D-deficient rats brought about the formation of the satellite CaBP forms from the chick protein. However, purified chick CaBP was unchanged by incubation in buffer alone for up to 2 h at 37 degrees C. 3) Placing rat kidney cytosols in a boiling water bath for 10 min inactivated the factor which generated the satellite forms as would be expected for an enzyme. The physiological significance of these forms of CaBP is as yet unknown.     

Human Germline Antibody Gene Segments Encode Polyspecific Antibodies

Structural flexibility in germline gene-encoded antibodies allows promiscuous binding to diverse antigens. The binding affinity and specificity for a particular epitope typically increase as antibody genes acquire somatic mutations in antigen-stimulated B cells. In this work, we investigated whether germline gene-encoded antibodies are optimal for polyspecificity by determining the basis for recognition of diverse antigens by antibodies encoded by three V_H gene segments. Panels of somatically mutated antibodies encoded by a common V_H gene, but each binding to a different antigen, were computationally redesigned to predict antibodies that could engage multiple antigens at once. The Rosetta multi-state design process predicted antibody sequences for the entire heavy chain variable region, including framework, CDR1, and CDR2 mutations. The predicted sequences matched the germline gene sequences to a remarkable degree, revealing by computational design the residues that are predicted to enable polyspecificity, i.e., binding of many unrelated antigens with a common sequence. The process thereby reverses antibody maturation in silico. In contrast, when designing antibodies to bind a single antigen, a sequence similar to that of the mature antibody sequence was returned, mimicking natural antibody maturation in silico. We demonstrated that the Rosetta computational design algorithm captures important aspects of antibody/antigen recognition. While the hypervariable region CDR3 often mediates much of the specificity of mature antibodies, we identified key positions in the V_H gene encoding CDR1, CDR2, and the immunoglobulin framework that are critical contributors for polyspecificity in germline antibodies. Computational design of antibodies capable of binding multiple antigens may allow the rational design of antibodies that retain polyspecificity for diverse epitope binding.