Heme reduction of ferric lactoperoxidase (LPO) into its ferrous form initially leads to the accumulation of the unstable form of LPO-Fe(II), which spontaneously converts to a more stable species, the two of which can be identified by Soret peaks at 440 and 434 nm, respectively. Our data demonstrate that both LPO-Fe(II) species are capable of binding O2 at a similar rate to generate the ferrous-dioxy complex. Its formation with respect to O2 was first order and monophasic and with rate constants of kon = 3.8 × 104 m–1 s–1 and koff = 11.2 s–1. The dissociation rate constant for the formation of LPO-Fe(II)-O2 is relatively high, in contrast to hemoprotein model compounds. This high dissociation rate can be attributed to a combination of effects that include the positive trans effect of the proximal ligand, the heme pocket environment, and the geometry of the Fe-O2 linkage. Our results have also shown that the decay of the LPO-Fe(II)-O2 complex occurs by two sequential O2-independent steps. The first step involves formation of a short-lived intermediate that can be characterized by its Soret absorption peak at 416 nm and may be attributed to the weakening of the Fe(II)-O2 linkage with a rate constant of 0.5 s–1. The second step is spontaneous conversion of this intermediate to generate the native enzyme and presumably superoxide as end products with a rate constant of 0.03 s–1. A comprehensive kinetic model that links LPO-Fe(II)-O2 complex formation to the LPO catalase-like activity, combined with the classic catalytic cycle, is presented here.

Great scientific discoveries rarely originate from small and poor countries. However, the lives and achievements of three Yugoslav scientists who were active in the biomedical sciences, Laza K. Lazarević (1851-1891), Ivan Djaja (1884-1957), and Pavao Stern (1913-1976), serve as an example of success in this environment. These scientists, as well as the majority of other successful investigators in small and poor countries, were trained in foreign and developed countries and, upon return, were given the freedom to start a selfdependent research program. They overcame many obstacles, including wars and civil unrests, to contribute significantly to certain medical fields. It is interesting that although a Jew, Stern was allowed to work during the World War II in Zagreb, which became capital of the so-called Independent State of Croatia, a puppet state under German control. Perhaps his good name among pharmacologists helped him to keep position during this tough period. Nowadays, new technologies requiring for biomedical research are rather expensive, and poor countries cannot afford to finance many scientists. Thus, selection of the most productive researchers is the challenge for those who finance scientific work.