Reactions of neutral amino acids plus Na(+) with a cationic amino acid transport system.

Before we could understand the transport interactions between the neutral and the basic amino acids, we needed to answer four questions: 1. To what extent do diamino acids actually react with transport systems in their neutral forms? First, a,fl-diamino propionic acid, pK; = 6.7, was shown to behave in transport almost entirely as a neutral amino acid, an analogue to serine and alanine in one neutral form, and to isoserine and palanine in the other [ 1,2]. Next, we found that (IL, y-diaminobutyric acid [3], although largely cationic in neutral solution, was also conspicuously reactive for transport as a neutral acid [4]. Finally it became clear that all the diamino acids, including lysine, were sufficiently reactive as neutral amino acids so that at high concentrations this reactivity could predominate over that with the cationic amino acid (Lp) transport system [6]. Accordingly, we were persuaded to use homoarginine as a test substrate for that system, taking advantage of (a) the exceedingly small extent to which the guanidinium group exists in its depronated form in neutral solutions, and (b) a high transport activity arising from the presence of a large apolar section in the sidechain.