Isotope-labeled pyrylium compound

1. A compound represented by the formula (I):

or a salt thereof.

2. The compound according to claim 1 , wherein the formula (I) has one or more carbon atoms having a mass number of 13, or a salt thereof.

3. The compound according to claim 1 , which is one compound selected from the group consisting of PyII-0, PyII-2, PyII-4, PyII-6, PyII-8, PyII-10 and PyII-12 represented by the formulas (II):

wherein carbon atoms shown by black balls have a mass number of 13, or a salt thereof.

4. A kit for quantifying an amino group-containing target substance in a biological sample by using a mass spectrometer, which comprises, as a labeling compound, two or more compounds having a mutually different mass due to isotope labeling, which are represented by the formula (I):

or a salt thereof.

5. The kit according to claim 4 , comprising two or more compounds represented by the formula (I) having a mass difference of not less than 6 or a salt thereof, wherein the target substance is a protein.

6. The kit according to claim 5 , comprising

a compound represented by the formula (I), which does not contain a carbon atom having a mass number of 13,

a compound represented by the formula (I), which has 6 carbon atoms having a mass number of 13, and

a compound represented by the formula (I), which has 12 carbon atoms having a mass number of 13,

or a salt thereof.

7. The kit according to claim 6 , comprising PyII-0, PyII-6 and PyII-12 represented by the formula (III):

wherein carbon atoms shown by a black ball have a mass number of 13.

8. The kit according to claim 4 , comprising two or more compounds represented by the formula (I) having a mass difference of two or more, or a salt thereof, wherein the target substance is an amino group-containing non-peptidic compound.

9. The kit according to claim 8 , comprising two or more compounds selected from the group consisting of

a compound represented by the formula (I), which does not contain a carbon atom having a mass number of 13,

a compound represented by the formula (I), which has 2 carbon atoms having a mass number of 13,

a compound represented by the formula (I), which has 4 carbon atoms having a mass number of 13,

a compound represented by the formula (I), which has 6 carbon atoms having a mass number of 13,

a compound represented by the formula (I), which has 8 carbon atoms having a mass number of 13,

a compound represented by the formula (I), which has 10 carbon atoms having a mass number of 13, and

a compound represented by the formula (I), which has 12 carbon atoms having a mass number of 13,

or a salt thereof.

10. The kit according to claim 9 , comprising two or more compounds selected from the group consisting of PyII-0, PyII-2, PyII-4, PyII-6, PyII-8, PyII-10 and PyII-12 represented by the formulas (II):

wherein carbon atoms shown by a black ball have a mass number of 13.

11. A method of quantitatively analyzing an amino group-containing target substance in two or more biological samples by using a mass spectrometer, comprising

(1) a step of preparing two or more biological samples to be subjected to an analysis,

(2) a step of labeling a target substance in samples prepared by using, as a labeling compound, two or more compounds having a mutually different mass due to isotope labeling, which are represented by the formula (I):

or a salt thereof, to confer a mass difference to the target substance between the samples,

(3) a step of preparing a mixture from all samples subjected to the labeling, and

(4) a step of subjecting the mixture to mass spectrometry, determining a presence ratio of the target substance in the mixture based on the ratio of peak intensity in mass spectrum of a target substance mutually having a mass difference due to labeling, and determining a quantitative ratio of the target substance between samples subjected to the preparation of the mixture, from the obtained presence ratio and the mixing ratio of the samples in step (3).

12. The method according to claim 11 , wherein the target substance is an amino group-containing non-peptidic compound.

13. The method according to claim 12 , wherein the labeling compound comprises two or more compounds represented by the formula (I) having a mass difference of 2 or more, or a salt thereof.

14. The method according to claim 13 , wherein the labeling compound comprises two or more compounds selected from the group consisting of

a compound represented by the formula (I), which does not contain a carbon atom having a mass number of 13,

a compound represented by the formula (I), which has 2 carbon atoms having a mass number of 13,

a compound represented by the formula (I), which has 4 carbon atoms having a mass number of 13,

a compound represented by the formula (I), which has 6 carbon atoms having a mass number of 13,

a compound represented by the formula (I), which has 8 carbon atoms having a mass number of 13,

a compound represented by the formula (I), which has 10 carbon atoms having a mass number of 13, and

a compound represented by the formula (I), which has 12 carbon atoms having a mass number of 13,

or a salt thereof.

15. The method according to claim 14 , wherein the labeling compound comprises two or more compounds selected from the group consisting of PyII-0, PyII-2, PyII-4, PyII-6, PyII-8, PyII-10 and PyII-12 represented by the formulas (II):

wherein carbon atoms shown by a black ball have a mass number of 13.

16. The method according to claim 11 , wherein

one of the samples prepared in step (1) is the internal standard sample containing a target substance at a known concentration, and

the determination of the presence ratio in step (4) comprises determining a ratio of a peak intensity of a target substance derived from each sample other than the internal standard sample and a target substance derived from the internal standard sample.

17. A method of quantitatively analyzing a protein in two or more biological samples by using a mass spectrometer, comprising

(1) a step of preparing two or more biological samples to be subjected to an analysis,

(2) a step of labeling a protein in samples prepared by using, as a labeling compound, two or more compounds having a mutually different mass due to isotope labeling, which are represented by the formula (I):

or a salt thereof, to confer a mass difference to the same protein between the samples,

(3) a step of preparing a mixture from all samples subjected to the labeling,

(4) a step of digesting the protein in the mixture with a protease to give a peptide,

(5) a step of subjecting the obtained peptide to mass spectrometry, and determining a presence ratio of the peptide in the mixture based on the ratio of peak intensity in mass spectrum of the peptide mutually having a mass difference due to labeling, and

(6) a step of identifying a protein from which the peptide having the determined presence ratio derives, and determining a quantitative ratio of the protein between samples subjected to the preparation of the mixture, from the presence ratio and the mixing ratio of the samples in step (3).

18. The method according to claim 17 , wherein the labeling compound comprises two or more compounds represented by the formula (I) having a mass difference of 6 or more, or a salt thereof.

19. The method according to claim 18 , wherein the labeling compound comprises

a compound represented by the formula (I), which does not contain a carbon atom having a mass number of 13,

a compound represented by the formula (I), which has 6 carbon atoms having a mass number of 13, and

a compound represented by the formula (I), which has 12 carbon atoms having a mass number of 13

or a salt thereof, and the protein in 3 biological samples is quantitatively analyzed.

20. The method according to claim 19 , wherein the labeling compound comprises PyII-0, PyII-6 and PyII-12 represented by the following formulas (III):

wherein carbon atoms shown by a black ball have a mass number of 13.

21. The method according to claim 17 , wherein

one of the samples prepared in step (1) is the internal standard sample obtained by mixing all other prepared samples, and

the determination of the presence ratio in step (5) comprises determining a ratio of a peak intensity of a peptide derived from each sample other than the internal standard sample and a peptide derived from the internal standard sample.

22. A method of producing a compound of the formula (I) or a salt thereof, comprising condensing 3-ethyl-3-pentanol or 3-ethyl-2-pentene with anhydrous propionic acid in the presence of an anhydrous acid to give a compound of the formula (I):

or a salt thereof.