Nicotinamide adenine dinucleotide (NAD) and, optionally, a reducing agent such as dithiothreitol or beta-mercaptoethanol, preferably dithiothreitol are added employing a molar ratio of NAD:Formula 1 acid within the range from about 500:1 to about 1500:1, preferably from about 900:1 to about 1200:1.