Haemoglobin inhibits GTP-hydrolysis and GDP/GTP-exchange activities of a low M(r) GTP-binding protein, ras p21.

Haemoglobin was observed to inhibit the GDP/GTP-exchange activity of ras protein (ras p21) by measurement of [3H]GDP-dissociation activity in time- and dose-dependent manners. Haemoglobin also inhibited the [32P]GTP-hydrolysis activity of ras p21 time- and dose-dependently. These inhibitory effects of haemoglobin were lost after incubation of haemoglobin at 80 degrees C for 3 min. Globin showed limited inhibition on the [32P]GTP-hydrolysis activity of ras p21, and haemin had no effect, indicating that the ternary tetrameric structure of haemoglobin is essential for the inhibitory effects on ras p21 activities. Methaemoglobin also inhibited both [3H]GDP-dissociation and [32P]GTP-hydrolysis activities of ras p21 in a very similar manner to that by haemoglobin. The obtained results strongly suggest that haemoglobin suppresses the physiological function(s) of ras p21 in vivo inhibiting both [32P]GTP-hydrolysis and GDP/GTP-dissociation of ras p21 in erythrocytes.