Oxygen effect in the radiolysis of proteins. Part 2. Bovine serum albumin

Radiolysis of bovine serum albumin under aerobic and anaerobic conditions was studied by SDS-polyacrylamide gel electrophoresis. After Coomassie Blue or Fast Green staining quantitative evaluations give information about the degradation processes of the protein. Under nitrogen the main reaction is the aggregation caused by covalent cross-links, which includes only a small portion of intermolecular S-S bridges. Under air the radiolysis leads to peptide chain scission, which is not a random process, but yields specific protein fragments. A mechanism for this fragmentation reaction is suggested. The radiation-induced broadening of the serum albumin peak is interpreted as being a result of intramolecular disulfide exchange. In contrast to lactate dehydrogenase the degradation of serum albumin is enhanced by oxygen, probably because of its low tryptophan content.     

Biochemical investigations of retinotectal adhesive specificity

The preferential adhesion of chick neural retina cells to surfaces of intact optic tecta has been investigated biochemically. The study uses a collection assay in which single cells from either dorsal or ventral halves of neural retain adhere preferentially to ventral or dorsal halves of optic tecta respectively. The data presented support the following conclusions: (a) The adhesion of ventral retina to dorsal tecta seems to depend on proteins located on ventral retina and on terminal β-N-acetylgalactosamine residues on dorsal tecta. (b) The adhesion of dorsal retina to ventral tecta seems to depend on proteins located on ventral tecta and on terminal β- N-acetylgalactosamine residues on dorsal retina. (c) A double gradient model for retinotectal adhesion along the dorsoventral axis is consistent with the data presented. The model utilizes only two complementary molecules. The molecule suggested to be concentrated dorsally in both retina and tectum seems to require terminal β-N-acetylgalactosamine residues for adhesion. Its activity is not affected by protease. A molecule fitting these qualifications, the ganglioside GM(2), could not be detected in a gradient, but lecithin vesicles containing GM(2) adhered preferentially to ventral tectal surfaces. The second molecule, concentrated ventrally in both retina and tectum, is a protein and seems capable of binding terminal β-N- acetylgalactosamine residues. One enzyme, UDP-galactose:GM(2) galactosyltransferase, has been found to be more concentrated in ventral retina than dorsal, but only by 30 percent.