Stability of vitamin E in long-term stored serum

Stability is a critical consideration in biochemical quantitative analysis, but there have been few comprehensive long-term stability studies for any analyte. Knowing whether an analyte is stable during storage is critical for large-scale epidemiological studies in which sample collection and analysis may be separated by a long interval of storage time (under a variety of possible conditions). Another situation in which knowledge of stability is important is in studies that use for analysis specimen collections established perhaps years ago for other unrelated studies in which the donors have been followed longitudinally. This type of ‘retrospective-prospective’ study has been especially useful in examining the relationship between micronutrient levels in serum and cancer incidence [1,2]. The micronutrients that are of most interest in current hypotheses of carcinogenesis or chemoprevention are those with antioxidant and free-radical scavenging activity, which include vitamins A and E. Though both are fat-soluble vitamins and strong antioxidants, serum levels of vitamin E depend directly on the vitamin E content of the diet, and a well-defined nutritional syndrome for vitamin E deficiency has not been delineated. Much interest has been generated in recent years in studies linking the relationship of serum vitamin E levels and the incidence of cancer [2,6-lo], using prediagnostic stored serum. Studies in this laboratory established that vitamin A in serum is extremely stable during long-term storage, even at -20 “C and despite several freeze-thaw cycles [3,4]. There are several reports of long-term vitamin A instability [5,6], but these findings may be due to breakdown of vitamin A by lipid peroxides that build up during storage and are only released during extraction prior to analysis. This degradation of vitamin A may be eliminated by the addition of ascorbic acid to the extraction medium (not to the fresh serum before storage). Vitamin E is not as stable as vitamin A in serum during long-term storage, possibly because vitamin E does not have a specific binding protein like retinol binding protein. In this report we describe storage conditions in which vitamin E