Hexose transport in adipocytes

The co-transport mechanism of the coupled movement of D-glucose and Na+ across the brush-border membrane is now universally accepted (for a review, see Crane, 1977). The transport agency shows a stable asymmetry with respect to the plane of the membrane, as demonstrated by its asymmetric inactivation (or reactivation) by little-permeant or impermeant reagents. Comparison was made between littlepermeant versus very-permeant reagents and between sealed right-side-out membrane vesicles versus deoxycholate-extracted membrane fragments (Klip et al., 1979a,b, 1980a,b). The Na+/Dglucose co-transporter shows a functional asymmetry also [in addition, of course, to that which can be imposed on to it by asymmetric distributions of substrate(s) and/or by a Ay # 01. In fact, in right-side-out vesicles, (i) trans-inhibitions and trans-stimulations by substrates are much larger in influx than in efflux experiments, (ii) Dglucose influx responds to A I ~ much more than its efflux, (5) the apparent K , for efflux is more than one order of magnitude larger than the apparent K , for influx and (iv) influx and efflux rates may differ by one order of magnitude when measured at equivalent, but mirrored, conditions (M. Kessler & G. Semenza, unpublished work). The observed asymmetric insertion of the co-transporter in the membrane obviously agrees with current ideas on the biosynthesis and vectorial insertion of intrinsic membrane proteins. It rules out, also, ‘diffusing’ or ‘tumbling-over’ carrier models for its mode of operation, whereas it makes more likely a ‘gated channel’ (Semenza, 1981; see also Crane & Dorando, 1979, 1980), i.e., a ‘channel with multiple conformational states’, which is a limiting case of the general channel mechanism (Lauger, 1980). Little is known of the chemical structure of the intrinsic membrane protein(s) catdysing Na+/D-ghcose co-transport. It probably spans the membrane (Klip et al., 19796). It is endowed with an essential thiol group@) (either at the cytosolic or at the hydrophobic surface, but not at the substrate-binding site; Klip et al., 1979a, 1980a,b) and with an essential amino group(s) (J. Weber & G. Semenza, unpublished work). By photoaffinity labelling with azido-phlorizin, (a part of) the co-transporter has been identified with a protein band having, on sodium dodecylsulphate/polyacrylamide-gel electrophoresis, an apparent molecular weight of 72000 (Hosang et al., 1981; see also Klip et al., 1979~).