Sulfhydryl reagents proposals

The inhibition of hLAL by boronic acids and diethyl p-nitrophenyl phosphate (Sando and Rosenbaum, 1985 G. N. Sando and H. L. Brockman, unpublished, cited by Anderson and Sando, 1991) indicates that hLAL is a serine hydrolase. Two lipase/esterase consensus pentapep-tides, G-X-S-X-G, are found, but only one of them appears to be consistent with the packing requirements of the )8-eSer-a nucleophilic motif (see above). Susceptibility of the enzyme to sulfhydryl reagents, and the requirement of thiols for the stability of purified hLAL, prompted Anderson and Sando (1991) to propose that a cysteine residue, or rather a Cys/Ser couple, may be involved in an internal transacylation reaction. It must be pointed out, however, that hLAL has all three cysteines of the gastric enzyme (as well as six additional ones), and so the inhibitory Cys is also there. The same argument proposed herein with respect to hGL, i.e., that a free cysteine is topologically close to the active site, also holds for hLAL. 

Nitrilases have been studied less than the nitrile hydratases. The enzymes appear as homomultimers, exhibiting a wide range of molecular masses. The reaction mechanism depicted in Fig. 12.1-2 has been proposed recently by Kobayashi et al. 2S. Several nitrilases have been found to be inhibited by reagents which bind to thiol groups, indicating that sulfhydryl groups are essential for the catalytic activity of... 

The permeability coefficients of most mammalian red cell membranes to water under control conditions are normally 10 times higher than the corresponding value for lecithin cholesterol bilayers [30], In the presence of sulfhydryl-reactive reagents, the permeability coefficients of the two membranes are practically identical, which suggests that these reagents act by blocking the proposed aqueous pores leaving the lipid bilayer of the mammalian red-cell membranes as the only alternative route for water transport. 

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