What are the requisites for a model transport analog?

Perhaps because we have come to accept so well the possibility of exquisite precision in biomolecular recognition in general, and by enzymes in particular, a need has arisen for re-emphasis of the limits that may apply to the possible oneto-one specificity between the mediators and substrates for the special area of membrane transport. Early studies with bacteria had bro~gh,* attention to transport systems specific virtually for one particular amino acid, sugar, etc. Accordingly, early resistance was encountered to the now familiar idea of transport systems serving in parallel in mammalian cells and tissues, each system transporting a rather ~Jde range of amino acids. The consequence of this pattern is that each system handles a substantial range, although a circumscribed one, of amino acids, and that each amino acid is moved across the plasma membrane often by two or more mediating systems. Amino acid transport systems of rather wide scope havc subsequently been recognized in bacteria, and narrowly specific ones in higher animals. For the ubiquitous wide-range systems, an obvious need has arisen to discover for each transport system the substrate analog, natural or artificial, that would serve best to identify transport catalysed by that system.