Search

A new strain of laboratory rabbit, produced from Japanese White rabbit but characterized by its black eye and higher antibody titer, was used in a series of nitrogen (N) balance trials to investigate the requirement of crude protein (CP) for maintenance. The N balance was determined for growing (6–12 weeks) and finishing period (13–17 weeks). Thirty rabbits were used in each period. The rabbits in both periods were divided into five equal groups and fed the diets with equal digestible energy but different contents of CP (120, 140, 160, 180 and 200 g/kg DM). The N requirements for maintenance were estimated from the relationship between the N retention and N intake. Following the above series of N balance tests, 20 rabbits of 16 of age weeks were used with a N-free diet. The rabbits were divided into five groups and offered the N-free diet at levels of 55, 45, 35, 25 and 0 g/day, respectively. Net protein utilization ranged from 0.45 to 0.50 and increased with the advancing age of rabbits, but no significant difference was found between different contents of dietary CP. The estimated N requirement for maintenance was, on average, 485 mg/kg BW0.75 per day, equivalent to 3.03 g CP/kg BW0.75. The result from the trial with the N-free diet showed that N requirement for maintenance was 486 mg/kgBW0.75 per day, confirming the results obtained in the series of N balance tests. The lower intake of the N-free diet resulted in more N excretion suggesting that protein catabolism may occur in the body of rabbit to meet maintenance requirements for N when the dietary N intake was very low.

The mature peptide of Thermomonospora fusca xylanase A (TfxA) was successfully expressed in Pichia pastoris under the control of AOX1 promoter. The activity of recombinant T. fusca xylanase A (reTfxA) in culture supernatant was 117.3 ± 2.4 U/mg, which is 3 times higher than that of the native TfxA. The optimal temperature and pH for reTfxA were 60 °C and 6.0, respectively. When treated at 70 °C and pH 6.0 for 2 min, the residual activity of the reTfxA was 70%. The reTfxA was very stable over a wide pH range (5.0–9.0). After incubation over pH 5.0–9.0 at 25 °C for 1 h, all the residual activity of reTfxA was over 80%. The Km and kcat values for reTfxA were 2.45 mg/ml and 139 s−1, respectively. HPLC analysis revealed that xylobiose (X2) was the main hydrolysis product released from birchwood xylan and wheat bran insoluble xylan by reTfxA. Hydrolysis results of xylooligosaccharides showed that reTfxA was an endo-acting xylanase and xylobiose, xylotriose (X3), xylotetraose (X4), xylopentaose (X5), and xylohexaose (X6) could be hydrolysed. This is the first report on the expression of reTfxA in yeast and on the determining and quantifying of the hydrolysis products released from xylans and xylooligosaccharides by reTfxA.