title: Cationic composition and acid-base state of the extracellular fluid, and specific buffer value of hemoglobin from the branchiopod crustacean Triops cancriformis.
abstract: Recent insights into the allosteric control of oxygen binding in the extracellular hemoglobin (Hb) of the tadpole shrimp Triops cancriformis raised the question about the physico-chemical properties of the protein's native environment. This study determined the cationic composition and acid-base state of the animal's extracellular fluid. The physiological concentrations of potential cationic effectors (calcium, magnesium) were more than one order of magnitude below the level effective to increase Hb oxygen affinity. The extracellular fluid in the pericardial space had a typical bicarbonate concentration of 7.6 mM but a remarkably high CO(2) partial pressure of 1.36 kPa at pH 7.52 and 20 degrees C. The discrepancy between this high CO(2) partial pressure and the comparably low values for water-breathing decapods could not solely be explained by the hemolymph-sampling procedure but may additionally arise from differences in cardiovascular complexity and efficiency. T. cancriformis hemolymph had a non-bicarbonate buffer value of 2.1 meq L(-1) pH(-1). Hb covered 40-60% of the non-bicarbonate buffering power. The specific buffer value of Hb of 1.1 meq (mmol heme)(-1) pH(-1) suggested a minimum requirement of two titratable histidines per heme-binding domain, which is supported by available information from N-terminal sequencing and expressed sequence tags.
authors.affiliation: Institute of Zoophysiology, University of Münster, Münster, Germany. pirow@uni-muenster.de
authors.forename: Ralph
authors.initials: R
authors.lastname: Pirow
authors.forename: Ina
authors.initials: I
authors.lastname: Buchen
authors.forename: Marc
authors.initials: M
authors.lastname: Richter
authors.forename: Carsten
authors.initials: C
authors.lastname: Allmer
authors.forename: Frank
authors.initials: F
authors.lastname: Nunes
authors.forename: Andreas
authors.initials: A
authors.lastname: Günsel
authors.forename: Wiebke
authors.initials: W
authors.lastname: Heikens
authors.forename: Tobias
authors.initials: T
authors.lastname: Lamkemeyer
authors.forename: Björn M
authors.initials: BM
authors.lastname: von Reumont
authors.forename: Stefan K
authors.initials: SK
authors.lastname: Hetz
mesh_terms: D000818:Animals; D001639:Bicarbonates / Q000032:analysis; D002021:Buffers; D002245:Carbon Dioxide / Q000032:analysis; D002412:Cations / Q000032:analysis*; D003445:Crustacea / Q000235:genetics / Q000378:metabolism*; D015180:Electrophoresis, Gel, Two-Dimensional; D020224:Expressed Sequence Tags; D045604:Extracellular Fluid / Q000737:chemistry*; D006454:Hemoglobins / Q000235:genetics / Q000378:metabolism*; D006458:Hemolymph / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D011199:Potentiometry / Q000295:instrumentation / Q000379:methods; D013054:Spectrophotometry, Atomic
chemical_list: D001639:Bicarbonates; D002021:Buffers; D002412:Cations; D006454:Hemoglobins; D002245:Carbon Dioxide