Translocation of sugars into rat liver lysosomes. Evidence against a common carrier for d-glucose and d-ribose

Abstract

A conflict exists in the literature concerning the mode of translocation of D-glucose and D-ribose across the lysosome membrane. The more rapid net uptake of ribose, when measured by the osmotic-protection technique, has been attributed either to its smaller size and lower hydrogen-bonding capacity, or to a lower affinity for a transport system shared by both sugars. The latency of acid beta-hexosaminidase in isolated rat liver lysosomes was measured after preincubation for periods up to 1 h in various solutions containing glucose and/or ribose, and in some cases sucrose. After confirmation of the superior osmotic protection afforded by glucose (than by ribose), it was shown that a solution 0.125 M in both glucose and ribose provided protection intermediate between that given by 0.25 M-glucose and that given by 0.25 M-ribose. This result is inconsistent with the common-carrier hypothesis.

Lysosomal sugar transport: facilitated or free diffusion?

“…1, Table 1) that the behaviour of the mixture may or may not be intermediate between that of either sugar alone, depending on their Km values. The intermediate behaviour found in the experiments of Bird et al (1987) therefore cannot be used to argue for the absence of a lysosomal transport system for ribose and glucose.…”

“…: a reply The main point at issue is not whether a monosaccharide transporter exists in the rat liver lysosome membrane, but the mechanism(s) by which sugars cross this membrane. In our recent paper (Bird et al, 1987) we quoted both the evidence for a glucose carrier (evidence repeated in the Letter by Maguire et al, 1988) and also the evidence that some sugars and their derivatives cross by passive diffusion. In their most recent experimental paper on sugar translocation across the lysosome membrane, Maguire et al (1983) concluded (we believe wisely) that glucose crosses partly by facilitated and partly by passive diffusion.…”

“…The later direct study of unidirectional sugar uptake showed that D-glucose and D-ribose indeed compete for uptake (Maguire et al, 1983). Bird et al (1987) conclude that the simplest explanation of their data is that glucose and ribose enter lysosomes by simple diffusion. This is very difficult to accept without an explanation of our findings in the direct study, which, in addiiton to showing competition, showed the process to be sugar-specific and stereospecific, saturable and inhibitible by cytochalasin B with very little effect of cytochalasin E (Maguire et al, 1983).…”

“…These are such that, at high concentrations, the net rates of uptake of a series of sugars may, paradoxically, be in the reverse order of their affinities for the system (Stein, 1967). Bird et al (1987) attempted to test the hypothesis advanced by Hales in experiments using the osmotic protection method. This method only very indirectly measures sugar uptake.…”

Effects of phospholipase A2 on the lysosomal ion permeability and osmotic sensitivity

“…The latency of a lysosomal enzyme refers to the percent of intact lysosomes as revealed by the inability of substrate to reach the lysosomal enzyme until the organelles are deliberately ruptured (Greene and Schneider, 1992). 4-Methylumbelliferyl Nacetyl-␤-d-glucosaminide, the substrate of lysosomal ␤-hexosaminidase, was used at 1 mM to measure the enzyme activity (Bird et al, 1987). The liberated 4-methylumbelliferone was determined by measuring its fluorescence (excitation 365 nm, emission 444 nm) on a Hitachi F-4010 fluorescence spectrophotometer.…”