Synthesis of [alpha-methyltyrosine-4]angiotensin II: studies of its conformation, pressor activity, and mode of enzymatic degradation.

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Abstract
Modifications in angiotensin II and its antagonistic peptides that should have increased in vivo half-lives but not reduced biological activity were studied by determining the effect of .alpha.-methylation of the tyrosine in position 4. [.alpha.-Methyltyrosine-4]angiotensin II, synthesized by the solid-phase procedure, showed 92.6 .+-. 5.3% pressor activity of angiotensin II in rats. Incubation with .alpha.-chymotrypsin for 1 h indicated absence of degradation although, under the same conditions, angiotensin II was completely degraded to 2 components. Comparison of the 1H NMR spectra in aqueous solution and the circular dichroism spectra in trifluoroethanol of angiotensin II and [.alpha.-methyltyrosine-4]angiotensin II suggested that .alpha. methylation of the tyrosine residue in angiotensin II does not lead to major changes in the overall solution conformation. These results are in contrast to those obtained with N-methylation in position 4, which drastically reduced the biological activity and produced remarkable changes in the peptide backbone and a severe limitation in rotational freedom of the side chains in tyrosine. It may be possible to synthesize potent angiotensin II analogs that have greater resistance to enzymatic degradation by .alpha.-methylation in position 4 (or 5) and simultaneous suitable modification at the NH2 and COOH termini.