Preparative HPLC separation of underivatized amino acids for isotopic analysis

A method for stable carbon isotope measurement of underivatized individual amino acids by multi-dimensional high-performance liquid chromatography and elemental analyzer/isotope ratio mass spectrometry

RATIONALE

To achieve better precision and accuracy for δ¹³ C analysis of individual amino acids (AAs), we have developed a new analytical method based on multi-dimensional high-performance liquid chromatography (HPLC) and elemental analyzer/isotope ratio mass spectrometry (EA/IRMS). Unlike conventional methods using gas chromatography, this approach omits pre-column chemical derivatization, thus reducing systematic errors associated with the isotopic measurement.

METHODS

The separation and isolation of individual AAs in a standard mixture containing 15 AAs and a biological sample, spear squid (Heterololigo bleekeri) were performed. AAs were isolated using an HPLC system equipped with a reversed-phase column and a mixed-mode column and collected using a fraction collector. After the chromatographic separation and further post-HPLC purification, the δ¹³ C values of AAs were measured by EA/IRMS.

RESULTS

The complete isolation of all 15 AAs in the standard mixture was achieved. The δ¹³ C values of these AAs before and after the experiment were in good agreement. Also, 15 AAs in the biological sample, H. bleekeri, were successfully measured. The δ¹³ C values of AAs in H. bleekeri varied by as much as 30‰ with glycine being most enriched in¹³ C.

CONCLUSIONS

The consistency between the δ¹³ C values of reference and processed AAs demonstrates that the experimental procedure generates accurate δ¹³ C values unaffected by fractionation effects and contamination. This method is therefore suitable for δ¹³ C analysis of biological samples with higher precision than conventional approaches. We propose this new method as a tool to measure δ¹³ C values of AAs in biological, ecological and biogeochemical studies.

Improving yields of deuterated, methyl labeled protein by growing in H2O

Solution NMR continues to make strides in addressing protein systems of significant size and complexity. A fundamental requirement to fully exploit the ¹⁵N-¹H TROSY and ¹³C-¹H₃ methyl TROSY effects is highly deuterated protein. Unfortunately, traditional overexpression in Escherichia coli (E. coli) during growth on media prepared in D₂O leads to many difficulties and limitations, such as cell toxicity, decreased yield, and the need to unfold or destabilize proteins for back exchange of amide protons. These issues are exacerbated for non-ideal systems such as membrane proteins. Expression of protein during growth in H₂O, with the addition of ²H-labeled amino acids derived from algal extract, can potentially avoid these issues. We demonstrate a novel fermentation methodology for high-density bacterial growth in H₂O M9 medium that allows for appropriate isotopic labeling and deuteration. Yields are significantly higher than those achieved in D₂O M9 for a variety of protein targets while still achieving 75-80% deuteration. Because the procedure does not require bulk D₂O or deuterated glucose, the cost per liter of growth medium is significantly decreased; taking into account improvements in yield, these savings can be quite dramatic. Triple-labeled protein is also efficiently produced including specific ¹³CH₃ labeling of isoleucine, leucine, and valine using the traditional ILV precursors in combination with an ILV-depleted mix of ²H/¹⁵N amino acids. These results are demonstrated for the membrane protein sensory rhodopsin II and the soluble proteins human aldoketoreductase AKR1c3, human ubiquitin, and bacterial flavodoxin. Limitations of the approach in the context of very large molecular weight proteins are illustrated using the bacterial Lac repressor transcription factor.