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Ligand size is a major determinant of high-affinity binding of fucose- and galactose-exposing (lipo)proteins by the hepatic fucose receptor.

ABSTRACT: Previous in vivo studies have demonstrated that small galactose-exposing particles are preferentially internalized by the asialoglycoprotein receptor on the parenchymal liver cell and large particles by the galactose-particle receptor on the Kupffer cell. In this study, we have investigated using in vitro binding studies whether the affinity for either receptor is affected by the ligand size. The asialoglycoprotein receptor appeared to bind and process lactosylated proteins irrespective of their size. In contrast, recognition of galactose-exposing proteins by the galactose-particle receptor on the Kupffer cell was strongly dependent on size. The affinity increased 3000-fold with protein sizes increasing from 5 to 15 nm, reaching its maximum at approx. 1 nM for ligands larger than 15 nm. Apparently, the preferential in vivo uptake of large galactose-exposing ligands by Kupffer cells does not result from an inability of the parenchymal liver cells to internalize these ligands, but from the high affinity of large ligands for the galactose-particle receptor and the strategic anatomical localization of the Kupffer cells in the liver. In the preceding paper [Kuiper, Bakkeren, Biessen and Van Berkel (1994) Biochem. J. 299, 285-290] the galactose-particle receptor on the Kupffer cell was suggested to be identical with the fucose receptor. 125I-Lac-LDL-binding studies clearly showed that the galactose-particle receptor exhibited high-affinity binding of fucose-exposing proteins also. The affinity of fucosylated proteins for the galactose-particle receptor was greatly affected by ligand size. The above data strongly support the hypothesis that the galactose-particle receptor is identical with the fucose receptor. The size of neoglycoproteins can be appreciated as a new major determinant of affinity for the fucose receptor.

SUBMITTER: Biessen EA

PROVIDER: S-EPMC1138053 | biostudies-other | 1994 Apr

REPOSITORIES: biostudies-other

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Project description:Sponges play an important ecological function in many benthic habitats. They filter large volumes of water, retain suspended particles with high efficiency, and process dissolved compounds. Nevertheless, the factors that regulate sponge pumping rate and its relation to environmental factors have been rarely studied. We examined, in situ, the variation of pumping rates for five Mediterranean sponge species and its relationship to temperature, particulate food abundance and sponge size over two annual cycles. Surprisingly, temperature and food concentration had only a small effect on pumping rates, and the seasonal variation of pumping rates was small (1.9-2.5 folds). Sponge size was the main determinant of the specific pumping rate (pumping normalized to sponge volume or mass). Within the natural size distribution of each species, the volume-specific pumping rate [PR V , ml min-1 (cm sponge)-3] decreased (up to 33 folds) with the increase in sponge volume (V, cm3), conforming to an allometric power function (PR V = aVb ) with negative exponents. The strong dependence of the size-specific pumping rate on the sponge size suggests that the simplistic use of this value to categorize sponge species and predict their activity may be misleading. For example, for small specimens, size-specific pumping rates of the two low-microbial-abundance (LMA) species (allometric exponent b of -0.2 and -0.3) were similar to those of two of the high-microbial-abundance (HMA) species (b of -0.5 and -0.7). However, for larger specimens, size-specific pumping rates were markedly different. Our results suggest that the pumping rate of the sponges we studied can be approximated using the measured allometric constants alone in conjunction with surveys of sponge abundance and size distribution. This information is essential for the quantification of in situ feeding and respiration rates and for estimates of the magnitude of sponge-mediated energy and nutrient fluxes at the community level. Further work is required to establish if and to what extent the low seasonal effect and the strong size dependency of pumping rate can be generalized to other sponges and habitats.

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Ligand size is a major determinant of high-affinity binding of fucose- and galactose-exposing (lipo)proteins by the hepatic fucose receptor.

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