No abstract

The solution structure of a thermostable cytochrome c-552 from a thermophilic hydrogen oxidizing bacterium Hydrogenobacter thermophilus was determined by proton nuclear magnetic resonance spectroscopy. Twenty structures were calculated by the X-PLOR program on the basis of 902 interproton distances, 21 hydrogen bonds, and 13 torsion angle constraints. The pairwise average root-mean-square deviation for the main chain heavy atoms was 0.91 +/- 0.11 A. The main chain folding of the cytochrome c-552 was almost the same as that of Pseudomonas aeruginosa cytochrome c-551 that has 59% sequence identity to the cytochrome c-552 but is less thermostable. We found several differences in local structures between the cytochromes c-552 and c-551. In the cytochrome c-552, aromatic-amino interactions were uniquely formed between Arg 35 and Tyr 32 and/or Tyr 41, the latter also having hydrophobic contacts with the side chains of Tyr 32, Ala 38, and Leu 42. Small hydrophobic cores were more tightly packed in the cytochrome c-552 because of the occupancies of Ala 5, Met 11, and Ile 76, each substituted by Phe 7, Val 13, and Val 78, respectively, in the cytochrome c-551. Some of these structural differences may contribute to the higher thermostability of the cytochrome c-552.

The intestinal absorption of small peptides was investigated in rats under unrestrained conditions. The peptide utilized in the experiment was hydrolysate of egg white protein with an average molecular weight of about 350 and containing less than 10% of free amino acids. We compared the intestinal absorption of these small peptides with that of an amino acid mixture with the same small peptide amino acid composition by determining the concentration of individual amino acids in portal blood after a single administration of a nitrogen source. The absorptive intensity of each amino acid was calculated from its rate of elevation in the portal blood; it was higher in the small peptides. The proportion of the amount of each amino acid absorbed in portal blood from small peptides was much more like the composition of the administered amino acids than like that from the amino acid mixture. Among the amino acids administered in the mixture, some amino acids such as L-tyrosine, L-threonine, L-serine and L-histidine increased more slowly in the blood than others. These results suggested that the small peptide formula was utilized more effectively than the amino acid mixture and had higher nutritive value.