Uptake of low molecular weight fractionated [3H]heparin by rat hepatocytes in the primary culture.

The uptake of low molecular weight fractionated [3H]heparin (LMWFH: 7000 Da) was examined, for comparison with that of high molecular weight fractionated [3H]heparin (HMWFH:20000 Da), in a primary culture of rat hepatocytes. The uptake of LMWFH increased almost linearly with time up to 60 min (extended uptake), although a faster uptake was observed in the initial 2 min (initial uptake). Both the initial and extended uptake were saturable, and the maximum uptake velocity (Vmax) and the Michaelis constant (Km) were estimated to be 10.7 pmol/min/mg protein and 398 nM, respectively, for the initial uptake and 0.34 pmol/min/mg protein and 116 nM, respectively, for the extended uptake. The Km for the extended uptake was 5 times larger than that of 21 nM for HMWFH, but the other parameters were comparable with those for HMWFH. Thus, an increase in Km, or a decrease in the apparent affinity, with a decrease in molecular weight in the extended uptake may be responsible for the reported lower hepatic uptake of low molecular weight heparin, compared with unfractionated heparin. It was also shown that both the initial and the extended uptake of LMWFH were inhibited by several analogs of heparin, including HMWFH, and anionic compounds such as 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), suggesting that LMWFH and HMWFH, in spite of a large difference in the molecular weight, share the same specialized uptake mechanism, in which an anionic moiety and/or heparin-like structure plays an important role.