Characterization of liver prenyl transferase and its inactivation by phenylglyoxal

The two interconvertible forms of pig liver prenyl transferase, A and B, consist of two identical subunits of Mr = 38 500 and are dimers. Form A contains six titratable SH-groups, whereas form B contains only four per dimer. The amino acid composition of the two forms is otherwise identical. Both enzyme forms are inactivated by phenylglyoxal. The inactivation in the absence of Mg2+ or Mn2+ is biphasic, each phase following pseudo first-order kinetics and is accompanied by a proportional binding of [14C]phenylglyoxal to the protein. In the initial fast phase of inactivation (t1/2 = 9.6 min) the amount of [14C]phenylglyoxal bound to the enzyme extrapolated to 1.1 arginyl residues and in the second phase (t1/2 = 23 min) to 2.2 arginyl residues modified per subunit for complete inactivation. 1 mM Mg2+ and 0.1 mM Mn2+ abolished the initial fast rate of inactivation and reduced its rate to a single half-life of about 60 min. Even at this slow rate of inactivation in the presence of Mg2+, the amount of [14C]phenylglyoxal bound to the enzyme extrapolated to about 2.3 arginyl residues modified per subunit for complete inactivation. In the absence of Mg2+ or Mn2+ only 1 mM geranyl pyrophosphate protected the enzyme against inactivation. However, in the presence of 1 mM Mg2+, isopentenyl, dimethylallyl and geranyl pyrophosphates gave additional protection over that observed with the metal ions, geranyl pyrophosphate being the most effective at 0.1 mM concentration.