Bioactivation of selenocysteine Se-conjugates by a highly purified rat renal cysteine conjugate beta-lyase/glutamine transaminase K

Selenocysteine Se-conjugates have recently been proposed as potential prodr ugs to target pharmacologically active selenol compounds to the kidney. Alt hough rat renal cytosol displayed a high activity of beta-elimination activ ity toward these substrates, the enzymes involved in this activation pathwa y as yet have not been identified. In the present study, the possible invol vement of cysteine conjugate beta-lyase/glutamine transaminase K (beta-lyas e/GTK) in cytosolic activity was investigated. To this end, the enzyme kine tics of 15 differentially substituted selenocysteine Se-conjugates and 11 c ysteine S-conjugates was determined using highly purified rat renal beta-ly ase/GTK. The results demonstrate that most selenocysteine Se-conjugates are beta-eliminated at a very high activity by purified beta-lyase/GTK, implic ating an important role of this protein in the previously reported beta-eli mination reactions in rat renal cytosol. As indicated by the rapid consumpt ion of alpha-keto-gamma-methiolbutyric acid, purified beta-lyase/GTK also c atalyzed transamination reactions, which appeared to even exceed that of be ta-elimination. The corresponding sulfur analogs also showed significant tr ansamination but were beta-eliminated at an extremely low rate. Comparison of the obtained enzyme kinetic data of purified beta-lyase/GTK with previou sly obtained data from rat renal cytosol showed a poor correlation. By dete rmining the activity profiles of cytosolic fractions applied to anion excha nge fast protein liquid chromatography and gel filtration chromatography, t he involvement of multiple enzymes in the beta-elimination of selenocystein e Se-conjugates in rat renal cytosol was demonstrated. The identity and cha racteristics of these alternative selenocysteine conjugate beta-lyases, how ever, remain to be established.