Simultaneous determination of protoporphyrin IX and magnesium protoporphyrin IX in Arabidopsis thaliana and Camellia sinensis using UPLC-MS/MS

BACKGROUNDS: Insertion of Mg(2+) into protoporphyrin IX (PPIX) to produce magnesium-protoporphyrin IX (Mg-PPIX) was the first step toward chlorophyll biosynthesis, which not only imparts plants green pigmentation but underpins photosynthesis. Plants that blocked the conversion of PPIX to Mg-PPIX displayed yellowish or albino-lethal phenotypes. However, the lack of systematic study of the detection method and the metabolic difference between species have caused the research on chloroplast retrograde signaling controversial for a long time. RESULTS: An advanced and sensitive UPLC-MS/MS strategy for determining PPIX and Mg-PPIX was established in two metabolic different plants, Arabidopsis thaliana (Columbia-0) and Camellia sinensis var. sinensis. Two metabolites could be extracted by 80% acetone (v/v) and 20% 0.1 M NH(4)OH (v/v) without hexane washing. Since the Mg-PPIX could be substantially de-metalized into PPIX in acidic conditions, analysis was carried out by UPLC-MS/MS with 0.1% ammonia (v/v) and 0.1% ammonium acetonitrile (v/v) as mobile phases using negative ion multiple reaction monitoring modes. Interestingly, it could be easier to monitor these two compounds in dehydrated samples rather than in fresh samples. Validation was performed in spiked samples and mean recoveries ranged from 70.5 to 916%, and the intra-day and inter-day variations were less than 7.5 and 10.9%, respectively. The limit of detection was 0.01 mg·kg(− 1) and the limit of quantification was 0.05 mg·kg(− 1). The contents of PPIX (1.67 ± 0.12 mg·kg(− 1)) and Mg-PPIX (3.37 ± 0.10 mg·kg(− 1)) in tea were significantly higher than in Arabidopsis (PPIX: 0.05 ± 0.02 mg·kg(− 1); Mg-PPIX: 0.08 ± 0.01 mg·kg(− 1)) and they were only detected in the leaf. CONCLUSIONS: Our study establishes a universal and reliable method for determining PPIX and Mg-PPIX in two plants using UPLC-MS/MS. This procedure will facilitate studying chlorophyll metabolism and natural chlorophyll production.