The Development of Awareness of Iron-Withholding Defense (1993)

THE DEVELOPMENT OF AWARENESS OF IRON-WITHHOLDING DEFENSE EUGENE D. WEINBERG* In William Shakespeare's tragedy King Lear, the Earl of Gloucester is attacked by the king's enemies who, in Act III, Scene 7, proceed to gouge out Gloucester's eyes. Gloucester's servant exclaims: "I'll fetch some flax, and whites of eggs,/To apply to his bleeding face." That raw egg white contains an anti-infective principle presumably was known to many persons of Shakespeare's day. Indeed, egg-laying creatures have been employing the principle for countless millenia. In 1944, microbiologists Arthur Schade and Leona Caroline observed that the antimicrobial action of the principle could be neutralized by iron [I]. The anti-infective factor subsequently was determined to be an 80 kDa ironbinding protein that is now termed ovotransferrin (OTf). The hen's egg, a perishable commodity in a semipermeable membrane within a fragile porous shell, is deposited in an environment heavily contaminated with bacteria, fungi, and protozoa. For thousands of years, hens successfully have employed the concept of ironwithholding defense to protect their precious legacy from infectious rot. The birds place a generous quantity of iron in the yolk for the developing embryo to use as a catalyst for DNA synthesis and for a variety of energy-yielding enzymatic systems. To prevent potential microbial invaders that migrate through the porous shell from obtaining lifeessential iron, the hens place none of the metal in the white, while including OTf as 12 percent of the solids. Moreover, OTf functions best in an alkaline milieu, thus our biochemically erudite birds adjust the pH of the white to 9.5. When Schade and Caroline observed the great potency and very broad spectrum of the antimicrobial activity of OTf, they predicted that additional examples of host iron-withholding defense would be found. *Department of Biology and Program in Medical Sciences, Indiana University, Bloomington, Indiana 47405.© 1993 by The University of Chicago. All rights reserved. 003 1-5982/93/3602-080110 1 .00 Perspectives in Biology and Medicine, 36, 2 ¦ Winter 1993 215 Within two years, they reported a similar iron-binding protein, siderophyllin , in human plasma [2]. This iron-binder, comprising 3.5 percent of our plasma proteins, is active at 7.4, the pH of normal plasma. Subsequently , Holmberg and Laurell noted that siderophyllin has a second function, namely, the transport of iron to host cells. To emphasize the second role, they changed the name of the protein to transferrin (Tf) [3]. In healthy humans, approximately 30 percent of the total ironbinding capacity of Tf is employed in transport of the metal and 70 percent in iron withholding. During episodes of microbial or neoplastic cell invasion, the iron-withholding capability can increase to as much as 90 percent [4]. A third example of host stationing of iron-binding proteins at potential sites of microbial invasion is that of lactoferrin (Lf). Initially observed in 1939 in human milk (in which it comprises 20 percent of the total protein), Lf was not purified and identified until 1960. During the decade of the 1960s, the protein was found in many other mammalian exocrine secretions: tears, nasal exudate, saliva, bronchial mucus, gastrointestinal fluid, hepatic bile, cervical mucus, and seminal fluid. Additionally, Lf is a major component of the secondary specific granules of circulating polymorphonuclear neutrophils (PMNs). The protein is released on degranulation of the cells in a septic area. Unlike OTf and Tf, iron binding by Lf can occur at pH values as low as 4. Thus Lf is a highly effective metal scavenger in septic sites in which the pH has been lowered by catabolic acids released from metabolically active microbial and/or defense cells. When the molecules of Lf become about 40 percent saturated with iron, they are assimilated by macrophages that have been attracted to the site of infection, and much of the metal is incorporated into the iron storage protein, ferritin. Induced Defensive Strategies Placement of Tf or Lf in nearly all of our various body fluids (with the notable exception of urine) constitutes a very effective first line of nonimmune defense against microbial invaders. Nevertheless, it also is necessary that an enhanced level of iron withholding be induced...