benzofurans and 5--(4-fluorosulfonylbenzoyl)inosine

Tyrosine residues 311 and 345 of the beta subunit of the bovine heart mitochondrial F1-ATPase (MF1) are present on the same peptide when the enzyme is fragmented with cyanogen bromide. Maximal inactivation of MF1 with 7-chloro-4-nitro[14C]benzofurazan [( 14C]Nbf-Cl) derivatizes tyrosine-311 in a single beta subunit. Cyanogen bromide digests of MF1 containing the [14C]Nbf-O-derivative of tyrosine-beta 311 were submitted to reversed-phase HPLC, with and without prior reduction of the nitro group on the incorporated reagent with dithionite. The retention time of the radioactive cyanogen bromide peptide was shifted substantially by reduction. When a cyanogen bromide digest of MF1 inactivated with 5'-p-fluorosulfonylbenzoyl[3H]inosine [( 3H]FSBI), which proceeds with derivatization of tyrosine-345 in a single beta subunit, was submitted to HPLC under the same conditions, the fragment labeled with 3H eluted with the same retention time as the [14C]Nbf-O-derivative before reduction. Doubly labeled enzyme was prepared by first derivatizing Tyr-beta 311 with [14C]Nbf-Cl and then derivatizing tyrosine-beta 345 with [3H]FSBI with and without reducing the [14C]Nbf-O-derivative of tyrosine-beta 311 with dithionite before modification with [3H]FSBI. The doubly labeled enzyme preparations were digested with cyanogen bromide and submitted to HPLC. The 14C and 3H in the cyanogen bromide digest prepared from doubly labeled enzyme not submitted to reduction eluted together. In contrast, the 14C and 3H in the digest prepared from doubly labeled enzyme which had been reduced eluted separately. From these results it is concluded that different beta subunits are derivatized when MF1 is doubly labeled with [14C]Nbf-Cl and [3H]FSBI.

Topics: Animals; Benzofurans; Cattle; Chromatography, High Pressure Liquid; Cyanogen Bromide; Inosine; Mitochondria, Heart; Proton-Translocating ATPases

The characteristics of ATP hydrolysis at a single catalytic site of the bovine heart F1-ATPase (MF1) as originally described by Grubmeyer et al. (Grubmeyer, C., Cross, R.L., and Penefsky, H.S. (1982) J. Biol. Chem. 257, 12092-12100) were compared with those of various chemically modified preparations of MF1 in which the steady state activity was severely attenuated. Although it was not necessary to age our preparations of native MF1 in the presence of 2 mM Pi to observe the same characteristics of single site catalysis, such aging did shift the equilibrium of bound substrate and bound products at the single catalytic site in favor of ATP. After loading a single catalytic site on the enzyme with substoichiometric [alpha,gamma-32P]ATP, the addition of 5-20 microM ATP or ADP was effective in promoting both the hydrolysis of bound [alpha,gamma-32P]ATP and release of radioactive products. Under these conditions, the 5-20 microM ATP added as promoter was hydrolyzed at a rate commensurate with the turnover rate of the enzyme, whereas the promoted hydrolysis of the [alpha,gamma-32P]ATP, preloaded at a single catalytic site, was considerably slower. Therefore, the high affinity, single catalytic site loaded first does not directly contribute to steady state ATP hydrolysis. That the single, high affinity catalytic site is not a "normal" catalytic site is supported by the properties of enzyme modified by 5'-p-fluorosulfonylbenzoyladenosine which exhibits only slightly altered characteristics of single site catalysis and promoted single site catalysis, despite exhibiting severely attenuated steady state turnover. Other modified forms of the enzyme in which the steady state activity was severely attenuated by derivatization with 5'-p-fluorosulfonylbenzoylinosine, 7-chloro-4-nitrobenzofurazan, or 1,5-difluoro-2,4-dinitrobenzene also bound substoichiometric ATP at a single catalytic site. However, the characteristics of single site hydrolysis by these modified forms of the enzyme differed considerably from those of native MF1.

Topics: Adenosine; Adenosine Triphosphate; Affinity Labels; Animals; Benzofurans; Binding Sites; Cattle; Dinitrofluorobenzene; Hydrolysis; Inosine; Mitochondria, Heart; Proton-Translocating ATPases; Structure-Activity Relationship