Use of Purified LyophilizedHuman Lactate Dehydrogenase Isoenzymes in a Study of the Measurement of Lactate Dehydrogenase Activity

We examinedthestability of human lactate dehydrogenase (EC 1.1.1.27; LD) isoenzymes 1, 2, and 3-purified to specific activities of about 200 kU/g-when lyophilized in a buffered stabilized matrix of bovine albumin. Each isoenzyme was prepared at two activityconcentrationsand stored at-20, 4, 20, 37, and 56 #{176}Cfor as long as six months. LD-1 activity decayed with zero-order kinetics, LD-2 and LD-3 with first-order kinetics. The extrapolated half-lives of these prep-arations at-20 #{176}Cvaried between 80 and 530 years. Stabil-ity of reconstituted samples stored at 4 #{176}Cwas excellent for LD-1 but poor for LD-2 and LD-3. We suggest that prepara-tions of human LD-1 be further investigated as a possible reference material. AddItIonal Keyphrases: enzyme activity potential reference material We recently described the purification, with good yield and specific activity, of lactate dehydrogenase (iAacta-te:NAD oxidoreductase, EC 1.1.1.27; LD) isoenzymes 1, 2, and 3 from human erythrocytes (1). We suggested that these LD isoenzymes might prove useful as reference or control materials to standardize LD methodology and to facilitate the intercomparability of LD values determined in different laboratories, as outlined by NCCLS (2-4). In serum of healthy humans, LD-2 is the single most prominent LD isoenzyme, accounting for 30 to 40 % of total LD activity, with LD-1, LD-2, and LD-3 together making up more than 75 % of the total serum LD activity. Therefore, any of these LI) isoenzymes would make relevant and appropriate refer-ence materials. We now report our experience in a six-month study of matrix-suspended preparations of LD-1, LD-2, and LD-3, regarding their suitability as possible reference materials for calibrating LD methodologies