Effects of modification of the sulfhydryl groups of calf lens low molecular weight alpha-crystallin

• Published in: Experimental eye research Vol. 26; no. 4; pp. 419 - 427
• Main Authors: Li, Lu-Ku, Spector, Abraham
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.01.1978
• Subjects: Calf lens; carboxymethylation; histidine modification; iodination; SH groups; size distribution; α-cristallin; iodination; SH groups; Calf lens; carboxymethylation; α-cristallin; histidine modification; size distribution
• ISSN: 0014-4835; 1096-0007
• DOI: 10.1016/0014-4835(78)90129-X

Chemical modification of 45–80% of the total sulfhydryl groups of the native low molecular weight calf α-crystallin with iodoacetic acid at pH 8·7 resulted in a marked decrease in size from 710 000 to 355 000 daltons, 19–125. Circular dichroic measurements between 200–260 nm of the carboxymethylated samples showed that the observed changes in size distribution are not accompanied by significant alteration of the polypeptide conformation. Since α-crystallin contains only four cysteines per 1000 residues, these results suggest that minor modifications in the protein can lead to dramatic changes in the aggregation of the α-crystallin macromolecule. That a maximum of 80% of the total SH groups of α-crystallin can be reacted in the absence of a dissociating medium indicates that the remaining 20% are blocked or located in the interior of the α-crystallin macromolecule. Iodoacetamide is less effective than iodoacetic acid in the carboxymethylation of the native protein, suggesting the possible involvement of positively charged groups near the accessible SH groups. Carboxymethylation of urea-treated α-crystallin or urea deaggregation and reaggregation of carboxymethylated alpha samples did not change the size of 12 S aggregates. These results are discussed with earlier data on the formation of 12 S aggregates by the treatment with urea or guanidine to suggest the architecture of the α-crystallin macromolecule. Carboxymethylation of some of the histidines has no effect on the size distribution of the α-crystallin while iodination of tyrosines results in an increase in size.