Are lysosomes involved in hexose transport regulation? Turnover of hexose carriers and the activity of thiol cathepsins are arrested by cyanate and ammonia.

The cycloheximide-related loss of transport activity (manifested as a decrease in Vmax for transport) in cultured Nil hamster fibroblasts was blocked by the addition of carbamoyl phosphate, cyanate (a product of spontaneous phosphate elimination from carbamoyl phosphate), or ammonium salts to the culture medium. Acid proteases capable of hydrolyzing alpha-N-benzoyl-D,L-arginine-beta-naphthylamine (cathepsins B1, H, and L) were also inhibited in situ by ammonia and cyanate. The inactivation of these cathepsins by ammonia was irreversible and probably was related to the increase in the intralysosomal pH known to be caused by an accumulation of ammonia in the lysosomes. The inhibition of the cathepsin activity by cyanate in situ (and in cell-free extracts) was completely reversible and blocked irreversible inhibition of the cathepsin(s) by N-ethylmaleimide. The inactivation of the cathepsins caused by cyanate was deduced to be the result of reversible blocking of sulfhydryl groups essential to the thiol cathepsin activity. The concomitant inhibition of thiol cathepsins and hexose carrier inactivation provided further evidence for the involvement of lysosomal proteases in at least part of the mechanism that regulates the rate of hexose transport in animal cells.