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The accumulated data indicate that a high level of homocysteine may be a central pathogenetic factor of chronic obstructive pulmonary disease. In this study, we investigated the effect of hyperhomocysteinemia (HM) on protein homeostasis in the rat lungs. The level of proteins, peptides, total proteolytic activity, as well as protein-peptide composition, were evaluated. HM was induced by daily intragastric administration of DL-homocysteine thiolactone (100 mg·kg-1 of body weight) to albino non-linear male rats for 28 days. Twelve hours after the last administration, the rats were sacrificed and the lungs were harvested. Our findings showed that HM caused the disturbances in the protein homeostasis in the lungs that are manifested by a decrease in the level of proteins in the young and old animals and an increase in the level of peptides in the rats of all studied groups. We found a change in the protein composition in the lung of HM rats - a decrease in the level of proteins with a molecular weight of 50 kDa to 100 kDa simultaneously with an increase in the level of proteins with a molecular weight of less than 50 kDa. Despite the fact that the peptide profile was the same in both control animals and HM animals, the level of individual peptide fractions increased significantly in the rats with HM. Obtained data could contribute to explain, at least in part, the mechanisms involved in the pathogenesis of lung damage in HM. [ABSTRACT FROM AUTHOR]