p50 is the oxygen partial pressure at which (a) respiratory flux is 50% of maximum oxygen flux, Jmax, at saturating oxygen levels. The oxygen affinity is indirectly proportional to the p50. The p50 depends on metabolic state and rate. (b) p50 is the oxygen partial pressure at which oxygen binding (on myoglobin, haemoglobin) is 50%, or desaturation is 50%.
Reference: Gnaiger 1998 J Exp Biol
MitoPedia concepts: MiP concept
MitoPedia topics: Substrate and metabolite