Measurement of prostate-specific antigen and human glandular kallikrein 2 in different body fluids

It has been demonstrated that prostate-specific antigen (PSA), in spite of its name, can be detected in body fluids and tumors from a variety of organ s. Investigations have shown that human glandular kallikrein 2 (hK2), a rel ated prostate-secreted protease, can activate the zymogen form of PSA, sugg esting that the two enzymes might work as a functional unit, with hK2 as th e activator molecule and PSA as the effector molecule. If this is true, the n hK2 should be found together with PSA in body fluids other than seminal p lasma, as well. Recently, a sensitive and specific assay was devised for hK 2, enabling its measurement in picogram quantities. With this assay, the co ncentration of hK2 was determined in samples of seminal plasma, amniotic fl uid, breast milk, and saliva. Simultaneously, the samples were assayed for molecular forms of PSA. In seminal plasma, the mean PSA concentration was 0 .82 mg/ml, while the hK2 level was around two orders of magnitude lower: me an value, 6.4 mu g/ml. Approximately the same ratio of PSA to hK2 as in sem inal plasma was found in amniotic fluid and breast milk, but in most sample s, the hK2 values were too low for direct measurements and had to be concen trated prior to analysis. Measurable levels of PSA, all in the free form, w ere detected in amniotic fluid at the thirteenth week of gestation and then gradually increased to levels around and over 1 mu g/L from the twentieth week. Significant levels of PSA were detected in amniotic fluid collected a t delivery, also. Measurable levels of mammary PSA were primarily detected in colostrum, with a range from less than 0.03 mu g/L to 2.1 mg/L. Around h alf of the molecules were in complex with protease inhibitor. Most surprisi ngly, determinations on saliva samples showed that none of them had detecta ble PSA levels but had measurable concentrations of hK2 with a mean value. 0.09 mu g/L. The presence in saliva suggests that hK2 can be the human equi valent to one of the mouse salivary kallikreins with important biological f unction, like the epidermal growth factor-binding protein or the gamma subu nit of nerve growth factor. However, this was ruled out, as a phylogenetic analysis showed that the human and mouse glandular kallikreins evolved inde pendently from a common precursor after the separation of the primate and r odent lineages. In conclusion, the measurements show that in addition to th e previously known secretion in seminal plasma, hK2 is secreted in amniotic fluid, breast milk, and saliva. Furthermore, the concerted expression of P SA and hK2 in seminal plasma, amniotic fluid, and breast milk suggests that the two proteases might form a functional unit but not always as demonstra ted by the sole presence of hK2 in saliva.