Ultraviolet-treated lipoproteins as a model system for the study of the biological effects of lipid peroxides on cultured cell. I. Chemical modifications of ultraviolet-treated low-density lipoproteins.

A new experimental model system constituted by ultraviolet-treated low-density lipoproteins (LDL) has been designed in order to investigate the biological effects of lipid peroxides entering the cell through the endocytotic pathway. This paper reports the chemical modifications of the lipid components and apolipoproteins of the ultraviolet-treated LDL. Human LDL were submitted to short ultraviolet radiations (254 nm, 0.5 mW/cm2, for variable periods of time) and compared to LDL peroxidized by iron. The lipid peroxidation was monitored by following the formation of the peroxidation products (conjugated dienes, thiobarbituric acid-reactive substances (TBARS) and fluorescent lipid-soluble products) and the change of the composition in polyunsaturated fatty acids, carotenes and vitamin E. Several parameters of the apo B-100 structure were investigated: molecular size (by SDS-PAGE) and TNBS-reactive amino groups (chemical determination by trinitrobenzene sulfonic acid). The most important feature was the absence of major modification of apo B-100 in ultraviolet-treated LDL: the molecular weight and the content in TNBS-reactive amino groups of apo B-100 were not modified. In contrast, iron-treated LDL exhibited a loss of the apo B-100 band and a decrease in the number of TNBS-reactive amino group. Both ultraviolet radiations and iron ions induced a significant decrease in the content of polyunsaturated fatty acids, carotenes and vitamin E together with a large formation of lipid peroxidation products. However, the time-course of the formation of conjugated dienes, TBARS and fluorescent lipid-soluble products was quite different using the two oxidative systems. These results demonstrate that ultraviolet radiations induced a strong peroxidation of the lipid content of LDL and no (or only minor) changes in the apolipoprotein moiety whereas iron-catalyzed peroxidation resulted in the formation fo lipid peroxidation products as well as apo B alterations.