Factors governing the stability of sperm whale, pig, and human metmyoglobin were examined by (1) measuring guanidinium chloride induced unfolding of apoglobins containing 22 replacements at positions 29(B10), 43(CD1), 64(E7), 68(E11), and 107(G8), (2) determining the rates of hemin loss from the recombinant holoproteins, and (3) estimating constitutive expression levels of the corresponding genes in Escherichia coli TB-1 cells. The denaturant titrations were analyzed in terms of a two-step unfolding reaction, N(native apoprotein)-->I(intermediate)-->U(unfolded), in which the intermediate is visualized by an increase in tryptophan fluorescence emission. Two key conclusions were reached. First, high rates of hemin loss are not necessarily correlated with unstable globin structures and vice versa. In general, both rates of hemin loss and the equilibrium constants for apoprotein unfolding must be determined in order to understand the overall stability of heme proteins and to predict the efficiency of their expression. Second, polar residues in the distal pocket cause marked decreases in the overall stability of apomyoglobin. Removal of hemin from V68N and L29N sperm whale myoglobins produces the molten globular I state at pH 7, 25 degrees C, without addition of denaturant. In contrast, the H64L and H64F mutations produce apoproteins which are 10-30 times more stable than wild-type apoglobin. The latter results show that protein stability is sacrificed in order to have the distal histidine (H64) present to increase O2 affinity and inhibit autooxidation.