Fuelgen, Robert Joachim

(b. Werden, Germany, 2 September 1884; d. Giessen, Germany, 24 October 1955)

biochemistry, histochemistry.

The son of a textile worker, Feulgen was educated in Werden, Essen, and Soest; in 1905 he entered the medical faculty of the University of Freiburg in Breisgau. For the completion of his training he went to Kiel, and while working in the city hospital he prepared his dissertation on the purine metabolism of patients afflicted with chronic gout. The years 1912-1918 were spent in Berlin at the Physiological Institute, the chemistry section of which was headed by the nucleic acid chemistyr Herman Steudel. Feulgen’s extension and criticism of Steudel’s work formed the subject of his Habilitationsschrift in 1919. The following year he was appointed to the physiological Institute at Giessen, where he spent the rest of his life, rising from assistant professor in 1923 to associate professor in 1927 and director of the Physiological-Chemical Institute in 1931.

In Berlin, Feulgen improved on Steudel’s extraction technique for thymonucleic acid(DNA), so that the product gave no biuret reaction and dissolved readily in water to give a colorless solution. By combining nucleic acid with congo red and malachite green, extracting the salts formed, and subjecting these to an elementary analysis, he believed he had obtained more reliable percentage compositions. His matching of the resulting nitrogen: phosphorus ratios with the ratios predicted from various molecular structures strengthened rather than shook his confidence in the tetranucleotide hypothesis.

Feulgen’s major discovery came in 1914, when he took up steudel’s observation (1908) of the reducing action of apurinic acid (then called thymic acid) on Fehling’s solution. Feulgen found that phenylhydrazine reacted with this acid, indicating the presence of aldehyde groups, and that Shif’s regent gave the magenta color indicative of furan. By the use of aldehyde blocking controls Feulgen was led to the correct conclusion that in the mild hydrolysis of thymonucleic acid to apurinic acid, loss of purines exposes aldehyde groups, which easily give rise to the furan structure. By treating thymonucleic acid with NHCI for ten minutes (optimum is pH 7) before application of Schiff’s reagent, he obtained the magenta color. Untreated thymonucleic acid failed to produce it, as did the RNA of yeast nucleic acid. It was already known that the carbohydrates in thymonucleic and yeast nucleic acid differ, the latter being a pentose sugar and the former probably a hexose sugar, in Feulgen’s opinion glucal—a compound discovered that year by Fischer—because it contains the furan structure, and he called his discovery the nucleal reaction.

In 1923, nine years after his discovery of the nucleal reaction, Feulgen applied it as a histochemical stain. In this way he was able to show that thymonucleic acid is found only in the nucleus and that both plant and animal cells give a positive nucleal reaction. Although he failed to detect the nuclei in yeast cells, he rightly concluded that the pentose nucleic acid of yeast is localized in the cytoplasm. At Feulgen’s first demonstration of these results, at the Congress of Physiology held at Tübeingen in 1923, Albrecht Kossel was impressed, but otherwise there was skepticism. By a through examination of the technique and by the use of aldehyde blocking, Feulgen established his test. In 1937 he succeeded in isolating rye germ nuclei that gave the nucleal reaction. This work effectively banished the old division of nucleic acids into the thymonucleic acids of animals and the yeast (pentose) nucleic acids of plants and established in its place the occurrence of both in the same cell. Although modern research has shown the presence of RNA in the nucleus and of DNA in the cytoplasm, the major part of these acids is still distributed in the way shown by Feulgen’s test.

In 1924 Feulgen and Voit discovered a positive nucleal reaction in the cytoplasm without previous mild hydrolysis. This “plasmal” reaction they showed to be due to aldehyde groups; and finding that lipide solvents negative the test, they concluded that a lipide precursor is responsible. In 1928 they isolated “plasmalogen,” and eleven years later they identified it as an acetal phosphatide.

Feulgen was orthodox in his acceptance of the tetranucleotide hypothesis, according to which thymonucleic acid is an oligonucleotide formed from a nucleotide of each of the bases thymine, cytosine, adenine, and guanine. Later he expressed reservations, and in 1936 it was clear to him that the undegraded material is a polymer and that the usual extractive procedures yield a mixture of depolymerized fragments. Studies of viscosity and optical activity gave him evidence of this change from what he termed the a form to the b form. He also discovered that conversion of a to b can be achieved by the action of a commercial preparation of the pancreatic juice. The depolymerizing enzyme that he believed to be present he called nucleogelase. From the same material M. McCarthy later obtained DNA depolymerase. These findings received little attention, and today Feulgen is remembered not for them but for his introduction of the nucleal reaction, which transformed nucleic acid cytochemistry. He was a skillful experimentalist whose certainly of the specific character of his nucleal reaction has been justified despite nearly thirty years of disputation over its nature and specificity.

BIBLIOGRAPHY

I. Original Works. Feulgen and his co-workers published seventy original papers, four contributions to biochemical textbooks, and a scholarly review of nucleic acid chemistry: “Chemie und Physiologie der Nucleinstoffe,” in A. Kanitz, ed., Die Biochemie in Einzeldarstellungen (Berlin, 1923). A complete list of his publications will be found in F. H. Kasten and in K. Felix (see below). The majority of his papers appeared in Hoppe-Seyler’s Zeitschrift für physiologische Chemie; the most important are “Über die ‘Kohlenhydratgruppe’ in der echten Nucleinsäure. Vorläufigen Mitteilungen,” 92 (1914), 154-158, announcing the nucleal reaction; “Mikroskopisch-chemischer Nachweis einer Nucleinsäure vom Typus der Thymonucleinsäure und die darauf beruhende elektive Färbung von Zellkernen in mikroscopischen Präparaten,” 135 (1924), 203-248, on the Feulgen stain, written with H. Rossenbeck; and “Die Darstellung der b-Thymoncleinsaure mittels der Nucleogelase,” 238 (1936), 105-110, The plasmal reaction was described in “Über einen weitverbreiteten Stoff (Plasmal, Plasmalogen), seinen histologischen Nachweis und seiner Beziehungen zum Geruch des gekochten Fleisches,” in Klinische Wochenschrift, 4 (1925), 1330, written with K. Voit.

II. Secondary Literature. The best scientific biography of Feulgen is K. Felix, “Robert Feulgen zum Gedächtnis,” in Hoppe-Seyler’s Zeitschrift für physiologische Chemie, 307 (1957), 1-13, with portrait and full bibliography. An English summary of this essay plus additional biographical data is in F. H. Kasten, “Robert Feulgen,” in W. Sandritter, ed., Hundred Years of Histochemistry in Germany (Stuttgart, 1964), pp. 97-101, also with portrait and bibliography. For brief details of Feulgen’s life see Leopoldina, 1 (1955), 52-53; and Poggendorff, VIIa, pt. 2 (Berlin, 1958), 31. The most authoritative account of the debate over the mechanism and specificity of the Feulgen reaction is A. G. E. Pearse, Histochemistry Theoretical and Applied, 2nd ed. (London, 1960), pp. 193-201.

Robert Olby