Determination of pyridoxamine 5'-phosphate in human blood plasma

Pyridoxamine 5'-phosphate in 18 microliters of human capillary blood plasma is determined by catalytic amplification using the apoenzyme of aspartate aminotransferase. Prior isolation from interfering substances is accomplished by employment of a cation exchange resin in batch operation. The procedure consists of the following stages. Stage I, denaturation of proteins. Trichloroacetic acid is used to precipitate plasma proteins and liberate any bound coenzyme. Dilute NaCl is added to expand the volume thus minimizing coenzyme entrapment in the precipitate. Stage II, isolation of the coenzyme. A sulfonated polystyrene ion exchange resin is used inside a centrifugal filter. Pyridoxamine 5'-phosphate in the supernatant from Stage I adsorbs to the resin. Pyridoxal 5'-phosphate, other organic phosphates, and Pi are removed by centrifugation. Rinsing with dilute NaBH4 destroys traces of pyridoxal 5'-phosphate and washes off residual inhibitors. Pyridoxamine 5'-phosphate is then desorbed with NaOH and Tris buffer and recovered by centrifugation. Stage III, reconstitution and assay. The desorbate from Stage II is incubated with excess apoenzyme. Specific activity of the reconstituted enzyme is measured. Interpolation from a standard curve relating enzyme specific activity and pyridoxamine 5'-phosphate concentration yields the plasma level of the cofactor. Approximately 3 h are required to carry out the procedure. Much of the coenzyme was found not be assayable if plasma was refrigerated overnight or if whole blood was left standing at room temperature for a few hours. The degradation was arrested with freezing at -80 degrees C. In a 13-day experiment involving a healthy subject, sharp rises of plasma pyridoxamine 5'-phosphate were found to occur in response to small doses of oral vitamin B6.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of adenosine-N6-methyl, propylthioether-N-pyridoxamine: an analog of a novel vitamin B6 tumor product

Studies on the time-course utilization of radiolabeled pyridoxine in rats with hepatomas led to the discovery of a novel vitamin B6 product. It is found in a spectrum of tumor lines but it is absent or occurs minimally in normal tissues. Hepatomas incorporate up to 20-30% of labeled pyridoxine into the novel product. Its structure was tentatively identified as adenosine-N6-methyl, propylthioether-N-pyridoximine-5'-PO4. However, results of tests on the incorporation of labeled precursors into the novel product by 3B3 hybridoma or HL-60 cells support an N6-diethylthioether bridge linking the adenosyl and pyridoxyl moieties. The synthesis of adenosine-N6-methyl, propylthioether-N-pyridoxamine is reported in this paper. The mass spectrum of this analog is similar to that of the tumor product as seen by its fragmentation in further support of the structure of the tumor product. Whether the latter may be part of tumor RNA is questionable. RNA was isolated for 3B3 or HL-60 cells after incubation with tritiated or 14C-pyridoxine using SDS-phenol repeated extractions in the presence of RNase inhibitors. Centrifugation of cRNA on 5-20% linear sucrose density gradients showed practically all the label at the top of the gradient. RNase treatment resulted in a labeled product which coeluted with the tumor product on reverse phase paired-ion HPLC and chromatographed as dinucleotide on paper. These results suggest that the novel tumor product may occur as a short oligonucleotide.