TREMATODE HEMOGLOBINS SHOW EXCEPTIONALLY HIGH OXYGEN-AFFINITY

Ligand binding studies were made with hemoglobin (Hb) isolated from tr ematode species Gastrothylax crumenifer (Gc), Paramphistomum epiclitum (Pe), Explanatum explanatum (Ee), parasitic worms of water buffalo Bu balus bubalis, and Isoparorchis hypselobagri (Ih) parasitic in the cat fish Wallago attu. The kinetics of oxygen and carbon monoxide binding show very fast association rates. Whereas oxygen can be displaced on a millisecond time scale from human Hb at 25 degrees C, the dissociatio n of oxygen from trematode Hb may require a few seconds to over 20 s ( for Hb Pe), Carbon monoxide dissociation is faster, however, than for other monomeric hemoglobins or myoglobins. Trematode hemoglobins also show a reduced rate of autoxidation; the oxy form is not readily oxidi zed by potassium ferricyanide, indicating that only the deoxy form rea cts rapidly with this oxidizing agent. Unlike most vertebrate Hbs, the trematodes have a tyrosine residue at position E7 instead of the usua l distal histidine, As for Hb Ascaris, which also displays a high oxyg en affinity, the trematodes have a tyrosine in position B10; two H-bon ds to the oxygen molecule are thought to be responsible for the very h igh oxygen affinity. The trematode hemoglobins display a combination o f high association rates and very low dissociation rates, resulting in some of the highest oxygen affinities ever observed.