Modification of low-density lipoprotein during radiolabeling with 99mTc using three labeling methods

The mechanisms of native low-density lipoprotein (LDL) uptake by monocytes and macrophages via the specific cholesterol down-regulated LDL-receptor differs form the mechanism responsible for the unregulated scavenging of the modified, for example, oxidized LDL, by the atherosclerotic plaques and foam cells. For this reason, we investigated if the 99mTc-labeled LDL stands for the native or modified molecule. The influence of the LDL sampling methods, isolation, preparation and radiolabeling of lipoproteins on structure modification and the subsequent imaging behavior has as yet not been addressed in detail.

Herein we present data on the effects of 99mTc labeling on some oxidation relevant parameters of LDL, such as the lag-time, thiobarbituric acid reactive substances (TBARS), relative electrophoretic mobility (REM), baseline dienes (BD), lipid peroxides (POX), free amino-groups (NH2-groups) and free sulphydryl-groups (SH-groups). Three methods of 99mTc labeling were compared: dithionite method (1), borohydride method (2) and ascorbic acid method (3). Data for oxidation parameters are expressed as a percent of freshly isolated native LDL (% native LDL) or as a percent of LDL treated with the labeling buffers and reagents, but in absence of the radioisotope (% control LDL).

The levels of BD were most influenced by methods 2 and 3, and remained almost unchanged when the method 1 was used. The lag-time of 99mTc-LDL produced by method 2 doubled but it was decreased by 23% when the method 3 was employed. No change in the lag-time compared to the native LDL was observed with the method 1. The TBARS levels were 3-5 fold higher than in native LDL when methods 1 and 2 were used, but 33% lower in products made by the method 3. The number of thiol groups increased 3 fold in method 1, was only slightly elevated in method 3, but reduced in method 2 compared to native LDL. NH2-groups were increased with all three labeling procedures, but this increase was not considered significant. REM was altered only in products obtained by methods 1 (1.5x increase) and by method 2 (1.25x increase). No fragmentation of Apo B using sodium dodecyl sulfate polyacrylamide gel (SDS-PAGE) electrophoresis was observed by 99mTc-LDL produced in any of the

The increase of lipid peroxide generation was observed only when the method 2 was used.

Of the three tested methods, we found all of them to render LDL oxidatively modified to some extent. Therefore, it appears that the native-LDL imaging with 99mTc-labeled LDL is impossible. Only the ascorbic acid method 3 appears to offer some protection and exerts antioxidant effects.