Porcine classification

Mammalian stratum comeum (SC) consists of highly comified cells embedded in a matrix of lipid bilayers (Matoltsy, 1976). These extracellular lipids are arranged in the form of multiple lamellae that are believed to constitute the major barrier to percutaneous penetration (Michaels et al., 1975 Elias, 1983). As discussed, the SC lipid membranes are made up predominantly of ceramides, cholesterol, free fatty acids, cholesteryl sulfate, and small amounts of some less well-defined nonpolar components (Gray et al., 1982 Yardley and Summerly, 1981). Six groups of ceramides have been characterized in porcine SC, as shown in Fig. 4 (Wertz et al., 1983). This classification was based on the polarity of the ceramides, with ceramide 1 being the least polar. 

Reaction with Diazo Reagents. The classification of the acid proteases on the basis of their pH optima as suggested by Hartley (43) was useful in the absence of other information on the nature of active site residues. Since Hartley s proposal, however, it has been discovered— first with porcine pepsin—that in the presence of Cu ions these enzymes can be specifically inactivated with diazotized dipeptide esters. An ester linkage is formed between one specific aspartic acid side chain and the inhibitor (50). 

The affinity differences for the diastereomers of PIA are often used as criteria for adenosine receptor subtype classification. The results summarized in Table III demonstrate that the diastereomeric selectivity for R- and S-PIA at Aj adenosine receptors is remarkably stable in a variety of radioligand binding and physiological assays. The largest difference in affinity was observed in the solubilized porcine atrial Aj adenosine receptor where R-PIA was approximately 43 fold more potent than S-PIA as an inhibitor of [1 I]HPIA binding. 

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