DIFFERENTIAL FEEDING-RELATED REGULATION OF UBIQUITIN AND CALBINDIN(9KDA) IN RAT DUODENUM

Analyses of the calcium-binding protein, calbindin(9kDa), purified to apparent homogeneity (SDS-PAGE) from rat duodenum, revealed variable c ontamination by two other 9 kDa proteins (up to 0.2 mol equivalent eac h) which were identified as ubiquitin and its C-terminal variant, des- Gly-Gly-ubiquitin. We found that the co-purification of these proteins did not reflect a tight molecular interaction but instead their unexp ectedly similar physical characteristics in nondenaturing conditions. Like calbindin(9kDa), free ubiquitin was abundant (1% and 0.4% of solu ble protein, respectively) in duodenum mucosa of 7-8-week-old rats and its concentration varied daily and with feeding status. In rats fed f rom midnight to 8.30 a.m., the ubiquitin concentration was specificall y higher at 10 pm than at 10 a.m. (11.2 +/- 0.7 and 7.7 +/- 0.8 nmol p er g wet weight, respectively, P < 0.02), whereas calbindin(9kDa) tend ed towards an opposite variation (18.0 +/- 1.9 and 21.8 +/- 1.7 nmol p er g, respectively). Based on its unusually high abundance and novel f eeding-related variations, ubiquitin must have an important functional role in the rat duodenum which is distinctly regulated from the calci um transport-associated role of calbindin(9kDa).