Characterization and role of polyphenol oxidase and peroxidase in browning of fresh-cut melon

Polyphenol oxidase (PPO) and peroxidase (POD) were extracted from two different varieties of melon ( Cucumis melo L. cantalupensis cv. Charentais and C. melo L. inodorus cv. Amarillo) and characterized using reliable spectrophotometric methods. In both cases the enzymes followed Michaelis-Menten kinetics, showing different values of kinetics parameters between the two cultivars: K m = 7.18 +/- 0.70 mM ('Charentais') and 6.66 +/- 0.20 mM ('Amarillo') mM; V max = 7.93 +/- 0.35 units/min ('Charentais') and 13.82 +/- 0.37 units/min ('Amarillo'), relative to PPO; K m = 24.0 +/- 2.10 mM ('Charentais') and 5.05 +/- 0.19 mM ('Amarillo') mM; V max = 344.83 +/- 10.32 units/min ('Charentais') and 80.64 +/- 2.01 units/min ('Amarillo'), relative to POD. Optimum pH for PPO was 7.0 for 'Charentais' and 7.5 for 'Amarillo, whereas it was 4.5 for both cultivars relative to POD. Melon PPO had maximum activity at 60 degrees C in both 'Charentais' and 'Amarillo' cultivars, whereas POD maximum activity was found at 45 degrees C in 'Charentais' and at 25 degrees C in 'Amarillo'. POD from both cultivars showed higher thermolability compared with PPO, losing >90% of relative activity after only 5 min of incubation at 70 degrees C. POD's activation energy was much higher than that of PPO (Delta E (#) = 86.3 and 160.6 kJ mol (-1) for 'Charentais' and 'Amarillo', respectively). PPO and POD activities from both cultivars showed a decreasing pattern as sugar concentration in the assay medium increased, except in POD extract from 'Charentais', which maintained its activity in the presence of high d-glucose concentration (up to 5 M). Changes in L*, a*, b*, chroma, and hue angle values were chosen to describe the browning development in the samples during storage at 5 degrees C. A slight decrease in L* value and a more marked reduction of a* value were noted in both cultivars above all at the end of storage period. POD activity during storage at 5 degrees C was highly correlated with changes of parameters a*, b*, chroma, and hue angle ( r (2) from 0.82 to 0.97) for cultivar 'Charentais'. According to these results, only POD activity seemed to be involved in browning of minimally processed melon.     

A transgenic mouse model of the chronic hepatitis B surface antigen carrier state

In an attempt to establish a model of the healthy carrier state in hepatitis B virus (HBV) infections, transgenic mice expressing HBV genes were produced. Fertilized one-cell eggs were microinjected with subgenomic fragments of HBV DNA containing the coding regions for the HBV surface antigen (HBsAg) and pre-S and X antigens. Either the normal (HBV) or metallothionein promoters were used to obtain expression of the HBV genes. There was no evidence of viral replication or tissue pathology. The integrated HBV DNA sequences were inherited in a normal Mendelian fashion. Three of 16 transgenic mice expressed HBV-encoded gene products to which they were immunologically tolerant. Expression was not tissue specific and may be influenced by the genomic integration site and cellular factors. Both HBsAg and pre-S antigen were detectable within the cytoplasm of hepatocytes and renal tubular epithelial cells. High serum concentrations of HBsAg were detectable and the secreted product appeared authentic as judged by mean density, morphology, mean particle diameter, polypeptide composition, and antigenicity. The absence of tissue pathology in these immunologically tolerant animals supports the hypothesis that cellular injury under these conditions is not a direct consequence of expression of the pre-S or HBs regions of the HBV genome.