Parvalbumin exists in two major forms, amounting to 2.5 and 3.2 g per kg of fresh muscle. 2. The composition divergence index between both forms, calculated from their amino acid composition, indicates 81% sequence identity; however both isoparvalbumins are immunologically distinct. 3. Beside Ca2+, perch parvalbumin binds 2 g atoms Mg2+ per mol with a dissociation constant of 10(-5) M. 4. Determination of the affinity for magnesium is of particular importance as there are indications that, in vivo, parvalbumin can remain in the Mg2+-state during the contraction-relaxation cycle instead of switching from the Ca2+-to the Mg2+-state and vice versa.
