Quantification of the accumulation and degradation of beta-very-low-density lipoproteins in vivo using a 19F-containing residualizing label and n.m.r. spectroscopy.

beta-Very-low-density lipoproteins (beta-VLDL)O were conjugated to the 19F-containing residualizing label, NN-dilactitol-3,5-bis(trifluoromethyl)benzylamine (DLBA), to determine whether the metabolism of this lipoprotein fraction could be characterized in vivo with n.m.r. spectroscopy. Solution state 19F high-resolution n.m.r. spectroscopy of DLBA-beta-VLDL, containing either intact apoproteins or selectively enzymically digested products, demonstrated that the extent of degradation could be distinguished by differences in spin-spin relaxation times (T2 times). DLBA-beta-VLDL was injected intravenously into rabbits, and accumulation of 19F in hepatic tissue was quantified non-invasively by n.m.r. spectroscopy 5 and 30 h after injection. In addition to quantifying the accumulation of DLBA-beta-VLDL in hepatic tissue, a marked decrease (approx. 100 Hz) in the linewidth of 19F resonance from labelled lipoproteins was observed at 30 h compared with the 5 h interval in continuously monitored animals. The change in linewidth was consistent with a decrease in molecular size that occurred during protein degradation, resulting in increased T2 times. To demonstrate that T2 times can be used as an index to quantify apoprotein degradation in vivo, relaxation measurements were performed on livers excised 20 h after injection of DLBA-beta-VLDL into rabbits. Two molecular motional fractions were revealed by relaxation profiles representing either an intact or an extensively degraded form of apoprotein. The amplitudes of each component were compared with results from trichloroacetic acid precipitation of liver homogenates acquired from rabbits 20 h after injection of beta-VLDL labelled with the radioiodinated analogue of DLBA, dilactitol-125I-tyramine. The amount of degraded apoprotein determined by n.m.r. spectroscopy and acid precipitation was 68.6 +/- 7.0% and 58.7 +/- 7.5% (n = 4) respectively. The results of this study demonstrate that 19F n.m.r. spectroscopy can be used to define the temporal characteristics of the hepatic metabolism of lipoproteins in vivo by quantifying both the tissue-specific accumulation and extent of apoprotein degradation. The methodology developed offers promise for the non-invasive, sequential and longitudinal evaluation of lipoprotein metabolism in vivo.