Mechanism of the inhibition of calmodulin-dependent neuronal nitric oxide synthase by flaxseed protein hydrolysates

This work was aimed at producing potential nutraceutical peptides from flaxseed protein hydrolysate that can bind to calmodulin (CaM) and inhibit the activity of CaM-dependent neuronal nitric oxide synthase (nNOS), an enzyme that has been implicated in some forms of human diseases. Flaxseed protein isolate was hydrolyzed with alcalase, and the resultant protein hydrolysate was passed through a 1000-Da M.W. cut-off membrane to isolate low-M.W. peptides. The permeate from the membrane was loaded onto a cation-exchange column, and adsorbed peptides were separated into fractions I and II that had a content of 42 and 51% basic amino acids, respectively. Kinetic analyses showed that both fractions were capable of binding to CaM, which led to reductions in the activity of nNOS; the inhibition constant (K _j ) was 5.97 and 2.55 mg/mL for fractions I and II, respectively. Double reciprocal plots showed that the mode of enzyme inhibition was mostly noncompetitive. Estimation of nNOS structure by fluorescence spectroscopy indicated that binding of the peptides to CaM led to a gradual unfolding of enzyme structure as levels of the fractions were increased. We concluded that the flaxseed protein-derived peptides may be used as ingredients for the formulation of therapeutic foods.