The effects of methylglyoxal-bis(guanylhydrazone) on spermine binding and transport in liver mitochondria

This study evaluated the effect of the anticancer drug methylglyoxal-bis( g uanylhydrazone) (MGBG) on the binding of the polyamine spermine to the mito chondrial membrane and its transport into the inner compartment of this org anelle. Spermine binding was studied by applying a new thermodynamic treatm ent of ligand-receptor interactions (Di Note et al., Macromol Theory Simul 5: 165-181, 1996). Results showed that MGBG inhibited the binding of spermi ne to the site competent for the first step in polyamine transport; the int eraction of spermine with this site, termed S-1, also mediates the inhibito ry effect of the polyamine on the mitochondrial permeability transition (Da lla Via et al., Biochim Biophys Acta 1284: 247-252, 1996). In the presence of 1 mM MGBG, the binding capacity and affinity of this site were reduced b y about 2.6-fold; on the contrary, the binding capacity of the S-2 site, wh ich is most likely responsible for the internalization of cytoplasmic prote ins (see Dalla Via et ai., reference cited above), increased by about 1.3-f old, and its binding affinity remained unaffected. MGBG also inhibited the initial rate of spermine transport in a dose-dependent manner by establishi ng apparently sigmoidal kinetics. Consequently, the total extent of spermin e accumulation inside mitochondria was inhibited. This inhibition in transp ort seems to reflect a conformational change at the level of the channel pr otein constituting the polyamine transport system, rather than competitive inhibition at the inner active site of the channel, thereby excluding the p ossibility that the polyamine and drug use the same transport pathway. Furt hermore, it is suggested that, in the presence of MGBG, the S-2 site is abl e to participate in residual spermine transport. MGBG also strongly inhibit s Delta pH-dependent spermine efflux, resulting in a complete block in the bidirectional flux of the polyamine and its sequestration inside the matrix space. The effects of MGBG on spermine accumulation are consistent with in vivo disruption of the regulator of energy metabolism and replication of t he mitochondrial genome. (C) 1999 Elsevier Science Inc.