Function and structure of rat hepatic coproporphyrinogen oxidase.

Rat hepatic coproporphyrinogen oxidase, the sixth enzyme in the heme biosynthetic pathway, was purified 1340-fold with a yield of 39.7%. To obtain the soluble enzyme, different methods were applied to disrupt mitochondria, with sonication giving the highest yield (85%). The minimum catalytic form of enzyme was a dimer with a molecular mass of 77 +/- 4 kDa. The existence of aggregated forms was possible since in fractions of gel filtration elution activity was observed with higher molecular mass. We determined a Stokes radius of 36.3 A, a sedimentation coefficient (S20,w) of 5.06 S, and frictional ratio of 1.29, suggesting a nearly globular shape of the protein. Regardless of the type of salt, high ionic strength inhibits the enzyme, probably modifying its native structure. Experiments with amino acid modifiers showed that histidine, arginine, and tryptophan are involved in the catalytic process. Non-ionic detergents and phospholipids activated the enzyme, probably because they reproduce its natural hydrophobic environment. The present study describes a simple method for the purification of rat liver coproporphyrinogen oxidase, introducing for the first time data on the structure and function of the protein in a tissue often used as a laboratory model in biological studies, and contributing to the study of human hereditary coproporphyria.

Role of histidine residues in the alpha-bungarotoxin binding site of the nicotinic acetylcholine receptor.

This paper studies the effect of histidine chemical modification of the membrane-bound acetylcholine receptor from Discopyge tschudii on its specific alpha-bungarotoxin binding. The acylating reagent ethoxyformic anhydride (diethyl pyrocarbonate, DEP), was used. DEP-treatment induces a loss of binding capacity, time and DEP-concentration dependent. After a 30 min period of derivatization with 2 mM final DEP-concentration, at pH 7.4, the decrease reaches 70%; the loss of binding capacity is faster at pH 7.4 than at pH 6.0, as expected, since the amount of unprotonated species is higher under the first condition. Moreover, when ethoxyformylation is carried out at different pH values, the most important neurotoxin binding decrease occurs between pH 6.0 and 8.0. Furthermore, ethoxyformylation reversion restores such capacity. Consistent with the modification of a binding site, the ethoxyformylation does not bear on the affinity but reduces the number of receptors. Ethoxyformylation in the presence of carbamylcholine shows some ligand protective effect. These results, as a whole, strongly indicate a relevant role for histidine residues at the alpha-bungarotoxin binding site of the nicotinic acetylcholine receptor.

Ethoxyformylation of histidine residues in equine growth hormone.

Reactivity of histidine residues in equine growth hormone to ethoxyformic anhydride was studied. The existence of two kinetically different sets was demonstrated: one of them including only the slow reacting histidine 169 (k = 0.164 min-1) and the other containing fast reacting histidines 19 and 21 (k = 0.892 min-1). A correlation between the decrease in the capacity to compete with 125I-labeled hormone for rat liver binding sites and the degree of ethoxyformylation of the fast group was found. Circular dichroism studies indicated no significant conformational changes in the protein with all three residues modified. These results fully agree with those obtained for bovine growth hormone which is further evidence supporting the vinculation of histidines 19 and/or 21 with the binding site of these hormones to their specific receptors.