A molecular mechanism for the biphasic effect of exogenous arachidonate on platelets

A haemoprotein (M _r 40 000) made up of two apparently identical subunits, and having Soret maximum at 405 nm, but displaying it mostly at about 410 nm or often at 415 nm in crude extracts owing to different bound small molecules, found in platelets of several species, has been purified from calf platelets. The purified protein could bind the stable PGH₂ analogue, U-46619, without co-operativity (h = 1.0 and half-maximal saturation concentration, S _0.5 = 10 μM) and arachidonate with co-operativity that increased with arachidonate concentration (h increasing from 2̃ to 4̃). S _0.5 of arachidonate was 1.5 μM. Arachidonate binding to the protein was accompanied by its oligomerization.