Molecular Basis of the Difference in Oxygen Affinity Between Maternal and Foetal Red Blood Cells in the Viviparous Garter Snake Thamnophis Elegans

Abstract

Molecular mechanisms that may explain why oxygen affinity is higher in foetal than in maternal red blood cells were studied in the viviparous garter snake, Thamnophis elegans (Baird and Girard). Foetal and adult haemoglobins were structurally indistinguishable, as demonstrated by native polyacrylamide gel electrophoresis (PAGE), sodium dodecyl sulphate PAGE, low pH/urea PAGE, and gel filtration column chromatography. Oxygen-binding studies of haemoglobin in the absence of organic phosphates showed that adult and foetal haemoglobins had relatively high affinities for oxygen, low Bohr coefficients, and Hill coefficients of about 4.0 at pH7.0. Adenosine triphosphate (ATP) lowered the oxygen affinity of the haemoglobins from about 3.6 to 9.6mmHg (lmmHg=133.3 Pa) at pH6.8. Maternal red cells contained more nucleoside triphosphate (NTP) (primarily ATP) than did foetal cells by about 0.9molNTP mol−1 haemoglobin tetramer. No 2,3-diphosphoglycerate was detected in the cells. Combined levels of magnesium and calcium were comparable in maternal and foetal red cells. Mean corpuscular haemoglobin concentrations (MCHC) in foetal red cells were about 79% of maternal values. There were no significant differences in maternal and foetal methaemoglobin levels. It appears that a difference in maternal and foetal red cell organic phosphate concentrations, and possibly MCHC values, rather than a difference in haemoglobin structures, explain why oxygen affinity is higher in foetal than in maternal red blood cells in T. elegans.