Probing the renin active site by collisional quenching of endogenous fluorescence

The structural and enzymatic aspects of renin are of great interest in hypertension research. In this paper, we examine the solution accessibility of the three tryptophan (Trp) residues of mouse submaxillary gland renin by solute collisional fluorescence quenching. Our studies indicate that there are two "classes" of Trp residues in renin: class I, a class of Trp residues which are at or near the surface of renin and fully accessible to the fluorescence quencher iodide; and class II, a class of Trp residues which are, for practical experimental conditions, totally inaccessible to the aqueous solution. The former class contains 2 Trp residues, while only a single Trp is identified in the latter class. The presence of a tetradecapeptide substrate or a nonhydrolyzable substrate analogue (peptide H-77) lowers the accessibility of iodide to the class I Trp residues. These data indicate that the class I Trp residues are at or near the peptide-binding site of renin. In addition, the finding that the class I Trp residues are quantitatively quenched more efficiently than the Trp model compound indole suggests that the environment of the class I tryptophans may be positively charged, and thus have a higher "local" concentration of iodide. These data, taken together with the available sequence and computer-generated three-dimensional structure of renin, permit us to speculate that the class I Trp residues are Trp-39 and Trp-300. This solution study of renin structure is discussed in light of the known information about renin catalysis and physiology.