Biochemist Choh Hao Li purified and characterized human growth hormone, ushering in a new era of advances in clinical medicine based upon an understanding of fundamental biochemistry.
The pituitary is a small gland lying underneath the lower surface of the brain. Controlled by the nearby brain region known as the hypothalamus, the pituitary gland secretes into the bloodstream many hormonal substances responsible for regulation of biological function. Growth hormone, a peptide or small protein, is one of these pituitary hormones responsible for proper growth and development in young animals, including humans, and for other complex processes in adults.
Choh Hao Li.
In a series of classic experiments in the 1920’s and 1930’s, Herbert McLean Evans had demonstrated the growth-promoting potential of crude pituitary extracts from dogs. By the early 1940’s, Choh Hao Li and Evans of the University of California, Berkeley, had succeeded in isolating and partially characterizing growth hormone from bovine pituitary glands. This work was particularly noteworthy because of the exceedingly small amounts of the hormone present in the diminutive pituitary and because of the creative use of classical methods and emerging technologies, such as electrophoresis (a method in which molecules are separated according to differences in electrical charge), to purify the hormone effectively.
Because insufficient levels of growth hormone in human infants lead to retarded growth, availability of growth hormone supplements is important in clinical medicine. Despite the basic importance of Li and Evans’s work with animal growth hormones, it had no impact on human medicine because animal growth hormones, except for those of the higher primates such as monkeys, do not stimulate growth in humans. Logically, then, it became vital to isolate the human growth hormone.
The reason that human growth hormone is effective in humans, while most animal growth hormones are not, is that the different growth hormones differ from one another in composition. These hormones are all small proteins, meaning that they are made of amino acids, which are the building blocks for all proteins. There are twenty amino acids commonly used to build proteins, and the exact sequence of amino acids throughout the protein precisely determines the shape and properties of the protein. Human growth hormone differs from other growth hormones in both size (number of amino acids) and sequence of amino acids. These physical differences not only lead to differences in biological activity but also lead to great difficulty in working with the various hormones, because the technology to distinguish among these types of molecular differences has been rudimentary.
In the late 1940’s and early 1950’s, following the successful isolation of growth hormone from bovine pituitary, a number of other growth hormones from a variety of species were extracted using similar methodology to that used by Li and Evans. Availability of these growth hormones allowed for the biological testing that established the inactivity of nonprimate growth hormones in primates. In 1956, Li and Harold Papkoff summarized the state of affairs in an article published in the American journal Science. Growth hormone, or somatotropin, from cattle is inactive in humans. Growth hormone from fish is inactive in rats but, not surprisingly, active in fish. Bovine hormone is inactive in monkeys.
Up to this time, none of these hormones had been thoroughly characterized, meaning that their amino acid compositions and sequences were unknown. Li’s work, however, had established some physical properties of the bovine growth hormone
Both human and monkey pituitary glands are rare items, so gifts of these tissues to Li were critical to the work’s success. Human tissue was provided from researchers in Stockholm, Sweden, while monkey pituitaries came from the United States pharmaceutical firm Eli Lily. These gifts suggest important points about the fundamental scientific enterprise. First, it is international in scope; and second, cooperation among scientists is common and widely accepted as ethical behavior. Although competition within the scientific community is also accepted as healthy to progress, cooperation and competition must coexist to provide a balance that fosters both individual accomplishment as well as benefit for the entire scientific enterprise.
Li and Papkoff used established techniques as well as new tools to effect the purification of growth hormone from these scarce tissues. Crude preparations were extracted from the raw tissues with calcium oxide. Treating the crude preparation with the salt ammonium sulfate precipitated growth hormone along with some other proteins. This process, known as salting out, takes advantage of solubility characteristics to produce partial purification of proteins. The technique, although of historical significance, is still widely used in the initial phases of protein purification.
Li then used a technique that was only beginning to take hold at the time. In this technique, biochemical samples were poured over a long column of chemical material known as an ion exchange column. The sample materials adhered to the column with differing strengths and were selectively released as salt solutions of increasing strength washed the column. The appropriate fractions were then treated further chemically and centrifuged (a process involving high-speed spinning) to separate the heavier, active material. The purified product was subjected to electrophoresis to verify its extremely pure status. Using the purified material, Li and Papkoff performed physical studies showing that the human and monkey growth hormones (the monkey preparation being treated in a parallel fashion) were significantly different in amino acid composition and molecular size from the growth hormones of other animals.
The final piece of standard chemical characterization of growth hormone awaited development of efficient technology for analyzing the exact sequence of amino acids in the hormone. Li and his colleagues published the sequence data in 1966. By 1970, Li succeeded in producing human growth hormone using the sequence information and new techniques of laboratory synthesis. The synthesis brought to fruitful conclusion more than forty years of revolutionary thinking and experimentation in peptide chemistry. Li’s work represents the power of scientific work that effectively utilizes the cutting edge of scientific design and method. Although many new approaches have arisen, these germinal studies were responsible for charting the way.
Choh Hao Li’s lifelong work with peptide hormones, and especially the work with human growth hormone, resulted in a cascade of diverse but related effects. The most obvious application of the human growth hormone work has been in clinical medicine, where availability of pure, highly characterized molecules has produced dramatic positive changes for growth hormone deficient people, usually children. Pure preparations are essential in minimizing untoward effects produced by contaminants or other pituitary hormones.
The pituitary gland is a complex organ containing many other peptide hormones and other substances. Other peptide hormones, present in even small quantities, may yield activities such as stimulation of the genitourinary tract which are inappropriate or even dangerous. Availability of pure human growth hormone is critical because of the negligible activity of other growth hormones in humans. These species differences represent general evolutionary relationships between structures of molecules from different organisms and activity of the molecules; however, these differences are particularly pronounced in the case of growth hormone. Insulin, for example, which is likewise a peptide hormone, shows considerable cross reactivity between species. Although human insulin is preferable for human use, insulin from pigs has been a standard preparation in human medical practice.
In a scientific sense, Li’s work may have even longer-ranging consequences. He, among others, pioneered the techniques now routinely used to study all proteins. Since proteins are indispensable structural and functional components of all living organisms and even entities on the edge of life such as viruses and prions, Li’s work touches nearly every area of modern biological science. In a more specific sense, unraveling the structural mysteries of growth hormone has led to a greater understanding of the pituitary, its relationship to the brain, most notably to the hypothalamic region, and a better appreciation of the control and dynamics of hormone function.
One of the clearest reasons for this increase in knowledge is that pure, well-understood biological molecules can be used as tools for probing the structures they come from and the structures they act upon. Better understanding of the pituitary gland has proved to be instrumental not only in exploring growth-related disorders such as acromegaly, which occurs because of growth hormone excess in adults, but also in exploring many other physiological situations, such as the control of sexual functions and reproduction and the regulation of hormone action in the adrenal glands.
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Ganong, William F. Review of Medical Physiology. 22d ed. New York: Lange Medical Books/McGraw-Hill Medical, 2005. Ganong concisely places growth hormone into its context as a vital agent of physiology. Contains a nice review of pituitary function and includes a bibliography of historical and modern importance. -
Li, Choh Hao, ed. Hormonal Proteins and Peptides. New York: Academic Press, 1973-1984. This collection of volumes on the peptide subcategory of hormones discusses developments in growth hormone research as well as in the now large number of other related substances. It shows clearly the dramatic contributions Li’s early discoveries made in paving the way for a new significant area of scientific medicine. Volume 4, 1979, is specifically devoted to growth hormone. -
_______, et al. “Human Pituitary Growth Hormone, XII: The Amino Acid Sequence of the Hormone.” Journal of the American Chemical Society 88 (May 5, 1966): 2050-2051. Chemists cannot even begin to suggest closure on the studies for a protein until its primary structure (that is, its amino acid sequence) is known. This paper provides that vital information while nicely documenting important applications of the emerging technology in protein chemistry. -
Li, Choh Hao, and Herbert M. Evans. “The Isolation of Pituitary Growth Hormone.” Science 99 (March 3, 1944): 183-184. This short but straightforward paper has great historical significance because it represents the first “clean” preparation of bovine growth hormone. The later work with human pituitaries depended heavily on the insights and experiences gained in nonhuman tissues. -
Li, Choh Hao, and Harold Papkoff. “Preparation and Properties of Growth Hormone from Human and Monkey Pituitary Glands.” Science 124 (December 18, 1956): 1293-1294. The dramatic findings with human and primate growth hormones, both so vital to the medical applications of this work, are presented in this coherent article. -
Maisel, Albert Q. The Hormone Quest. New York: Random House, 1965. This is one of those fun books that reads like a novel while expertly chronicling the history of hormone research, including the pituitary hormones like growth hormone. Although dated, its coverage of the early days of hormone research makes it worthwhile reading. Motivating to budding endocrinologists.
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