Sulfhydryl groups react with iodoacetic acid according to the following reaction: RSH + ICH2COO- → RSCH2COO- + H+ + I- . The group added to the sulfur atom is called a carboxymethyl group. A native protein is reacted with excess iodoacetic acid, whereupon two moles of carboxymethyl groups are found to be bound per mole of protein. In a second experiment a sample of the protein is completely unfolded and reacted with excess mercaptoethanol and then with excess iodoacetic acid. Ten moles of carboxymethyl groups are now found to be bound per mole of protein. How many disulfides does the native protein contain?