Possible interaction of haemoglobin with a low Mr GTP-binding protein, ram p25

Haemoblobin (Hb) was observed to inhibit the GDP/GTP exchange activity of a low M(r) GTP-binding protein, ram p25. Hb also inhibited the [32P]GTP-hydrolysis activity of ram p25. These inhibitory effects of Hb were lost after incubation of Hb at 80 degrees C for 3 min, indicating that the ternary tetrameric structure of Hb is essential for the inhibitory effects on ram p25 activities. Hb did not inhibit [35S]GTP gamma S-binding of nucleotide-free ram p25. Methaemoglobin (MetHb) also inhibited both [3H]GDP-dissociation and [32P]GTP-hydrolysis activities of ram p25 in a very similar manner to Hb. The results strongly suggest that Hb may suppress physiological function(s) of ram p25 in vivo by inhibiting both [32P]GTP-hydrolysis and [3H]GDP-dissociation of ram p25.