尿素

Supplementary experiments we r e carried out concerning the purification of the enzyme which catalizes the reduction of DPN with urea. Tris buffer was found to be the better extracting solvent than phosphate to obtain the preparation of higher activity, although no activity was sometimes detected. It was established that amounts of hydroxylapatite gel so far employed can be decreased to make the enzyme solution without loss of activity. Thus, the enzyme solution can be prepared as follows. Fresh liver was homogenized in 3 volumes of M/15 Tris buffer of pH 7.0 to 7.5 by a Waring blender. Phosphate buffer also can be employed in place of Tris. Homogenate was immersed in water bath of 35℃ for 30 min. under agitation and contrifuged at 6,000 r.p.m. for 10 min. at 0℃. Then enzyme was fractionated with acetone at -10° to -15℃, except that centrifuging was carried out at 0℃. Half a volume of cold acetone was added dropwise into supernatant. After 5 to 10 min., precipitates formed were centrifuged at 6,000 r.p.m. for 10 min. and discarded. Similary, supernatant was further mixed with half a volume of acetone and centrifuged at 10,000 r.p.m. for 10 min. after standing for 5 to 10 min. Precipitate deposited between 33 % and 55 % of acetone refers to "Acetone- B preparation " which had been employed in the preceding studies. In this case, precipitate was washed three times with cold 57 % acetone, dissolved in water, dialysed against running water for 3 hrs. and employed after centrifuging at 3,000 r.p.m. for 10 min. Enzyme fractionated with c o ld acetone was further purified by treating with hydroxylapatite gel at 0℃. Gel was prepared according to Tiselius and kept for more than several weeks in cold. Acetone-B precipitate was dissolved in 10 ml of water. Four to 5 times the amount of the precipitate of gel were added into this aqueous enzyme solution, stirred for 5 min. and centrifuged at 6,000 r.p.m. for 5 min. at 0℃. Deposited gel was mixed with 20 ml of 2 M sodium acetate which had been adjusted to pH 7.5 and stirred for 5 min. The mixture was centrifuged and the supernatant was discarded. Gel was again mixed with desired volumes of 4 M sodium acetate of pH 7.5, stirred well for 5 min. and centrifuged at 10,000 r.p.m. for 10 min. Supernatant containing the enzyme was dialysed against running water for 3 hrs. using cellophane tube after filtration. Thus, clear enzyme solution was obtained. When Tris buffer was employed for extracting the enzyme from the original liver tissues, almost colorless final enzyme solution can be obtained, whereas very pale yellow color was remained in the enzyme solution in the case of phosphate. This fact might be supported by comparing absorption spectra of both kind of the enzyme solutions. Enzyme solution has two absorption maxima at 275 mμ and 415 mμ. Additional two small peaks at 545 mμ and 580 mμ were also observed, if the enzyme solution had been purified from the liver extract with phosphate. This enzyme solution has much steeper main peak at 415 mμ than that of Tris-enzyme. These observations might account for the difference of color of the enzyme solutions. Ultraviolet peak at 275 mμ may be attributed to protein. In general, decrease of absorption was brought about by gel-treatment throughout the range of wave length estimated. Thus, disappearance of red color in the raw enzyme solution was explained. Since specific activity of the enzyme is generally refered to the enzymatic activity per protein, its increment by purification was also illustrated by the remarkable decrease in absorption at 275 mμ. Enzymatic activity was e s timated spectrophotometrically by determining the increase in absorption at 340 mμ, formation of DPNH, according to the foregoing paper. However, reaction mixture was slightly modified as to consist of the following composition ; 1 ml of the enzyme solution, 1 ml of 0.05 % DPN solution, 0.5 ml of 5 x 10^-3M urea solution, 0.5 ml of 0.2 M Tris buffer of pH 7.5 and 1 ml of H_2O to make a final volume 4 ml. According to experimental conditions, composition of the reaction mixture can be modified, total volume of 4 ml being kept by changing the volume of water. Absorption of the reaction mixture estimated at 0 min. and after 30 min. at 30℃ by Hitachi's spectrophotometer EPU-2, with light pass of 10 mm and difference was calculated. Since liver contains many kinds of DPN-conjugated enzyme systems, a little formation of DPNH can be observed, as had been emphasized so far, even by the purified enzyme. Therefore, control experiment containing no urea was also checked and the increase in -log T was subtracted from those in tests. Since, enzymatic activity of the preparation varies from one material to another, the increase in absorption per mg of protein-N was sometimes calculated, especially for comparison. Concerning the purified enzyme solution, some properties were investigated. However, any differences could not be established from those with acetone-B enzyme. DPN is reduced enzymatically in Tris, veronal or acetate buffer but not in phosphate buffer. Inhibiting effect of phosphate related to the concentration. Of course, enzymatic activity is approximately proportional to the amounts of enzyme or DPN and the reaction time. Rate of the enzymatic re-action, however, reaches to the maximum at 10^-4M of urea, independent of the activity of the preparations. Much amounts of urea did not promote nor repressed the DPNH formation. DPN is reduced enzymatically with urea at highest rate at 35℃ and pH 7.5, while the increases in -log T in blank at alkaline range are higher than those in neutral range. As had been established with acetone-Benzyme, opposed to the ammonium dehydrogenase, TPN is reduced by the purified enzyme too, in presence of urea. Comparison of the activity of the urea-enzyme with that of the ammonium dehydrogenase in many preparations which had been purified by the same procedure but from different liver material indicated that both activities are not directly proportional each other. This observation also suggests that the urea-enzyme may be different from the ammonium dehydrogenase, although the properties are very likely.