The stability of cattle rhodopsin and of its protein moiety opsin toward acids and alkalies and on aging was determined by two criteria: maintenance of absorption spectrum, and capacity to regenerate after exposure to light.
On storage at 3°C. at pH near neutrality, the absorption spectrum in the visible region may remain unchanged for as long as 6 months; but the regenerability progressively declines, at very different rates in different preparations. The cause of this decline has not been determined. It may involve denaturation at sites other than the retinene-protein bond, which by the evidence of the absorption spectrum remains intact.
Cattle rhodopsin maintains its absorption spectrum at any pH from 3.9–9.6 for at least an hour at 25–27°C. To both sides of this pH range the pigment bleaches, the extinction falling to half in 1 hour at pH 3.3 and 10.5.
The exposure of rhodopsin to light greatly increases the vulnerability of the product (opsin) to acids and bases. Opsin rapidly loses its capacity to regenerate rhodopsin to both sides of the range of pH 5.5–7.0. Half the regenerability is lost within 45 seconds at pH 3.4 and 9.1; and within 1 hour at pH 5 and 8.