Biochemistry of Amino Acids - A. Majster 1961

Natural Amino Acids
Other Natural Amino Acids
Miscellaneous Amino Acids Occurring in Nature

An amino acid with The properties of a methyl derivative of Proline has been isolated from apple peels; however, further research is required to definitively establish The Structure of this compound [309]. Data have recently been published on the presence of α-aminopimelic acid [310], α-amino-γ-hydroxypimelic acid [311], and γ-hydroxyglutamic acid [312] in certain plants. A selenium-containing amino acid was isolated from Astragalus pectinatus grown on soils with a high selenium content [313]. It is known that selenium appears in the Proteins of wheat grown on selenium-rich soil, likely substituting for sulfur in the cystine molecule [314]. According to preliminary, unconfirmed data, Escherichia coli synthesizes an amino analogue of pantoic acid: α-amino-, β,β′-dimethyl-γ-hydroxybutyric acid [315].

It has been suggested that carbamic acid is a component of proteins [316]. α-Aminotricarballylic acid (p. 88) has not been found in natural products, although the corresponding α-hydroxy and α-keto acids represent important metabolic intermediates. The presence of β-hydroxyglutamic acid in proteins was reported in an early study [317]. However, subsequent research refuted this assumption [318, 319]; chromatographic analysis definitively established that the originally described product did not contain β-hydroxyglutamic acid [320]. Norleucine was also previously believed to be a protein constituent [321–323], but this hypothesis has likewise been disproven by chromatographic evidence [324].