Hydrogen ion binding properties of tuna haemoglobins

Tunas are very active fish with a high aerobic capacity, but they also regu larly perform burst swimming with massive production of lactic acid. The pr esent study examines whether H+ buffering by tuna haemoglobin (Hb) is eleva ted to cope with metabolic acidoses (by analogy with the high buffer capaci ty of tuna white musculature) or whether the Hb-H+ binding properties resem ble other teleosts that have low buffer values and high Haldane effects. H titration of oxygenated and deoxygenated composite Hb from yellowfin tuna, skipjack tuna and bigeye tuna in 0.1 M KCl revealed low Hb-specific buffer values in all three tunas. Values at physiological pH were comparable to t hose reported in less active species such as carp and eel. The fixed acid H aldane effect was large (maximal uptakes of close to 4 mol H+ per mol Hb te tramer upon deoxygenation). Thus, the Hb-H+ binding properties of very acti ve tuna species correspond to other teleosts. Low Hb buffer values may be a pre-requisite for the regulation of red blood cell pH via Na+/H+ exchange. Approximately nine 'neutral' groups were titratable in tuna Hbs, suggestin g that two ol-amino groups and seven histidine residues are titrated within each tetramer. (C) 2001 Elsevier Science Inc. All rights reserved.