Kallikreins are proteolytic enzymes that constitute a subfamily of serine 
proteases. Novel kallikrein genes were cloned recently, and it was shown that 
the human kallikrein family contains 15 genes tandemly aligned on chromosomal 
locus 19q13.3-q13.4. Based on their altered expression in tumor cells, 
kallikreins may be involved in the pathogenesis and/or progression of cancer. 
Evidence is presented that certain kallikreins may be exploited as diagnostic 
cancer biomarkers. Although the function(s) of novel kallikreins is currently 
unknown, increasing evidence suggests that kallikreins may participate in 
regulatory enzymatic cascade(s). Elucidation of the function of novel 
kallikreins largely depends on the availability of active recombinant proteins. 
Here, the zymogen for kallikrein 13 was overexpressed in Pichia pastoris and 
biochemically characterized. It was shown that the kallikrein 13 zymogen 
displays intrinsic catalytic activity leading to autoactivation. A clipped form 
of kallikrein 13 was identified, indicating autocatalytic cleavage at the 
internal bond R114-S115. Mature kallikrein 13 displays trypsin-like activity 
with restricted specificity on synthetic and protein substrates. Combinatorial 
P1-Lys libraries of tetrapeptide fluorogenic substrates were synthesized and 
used for the profiling of the P2 specificity of selected kallikreins. 
Interestingly, it was shown that human kallikrein 13, similarly to PSA, could 
specifically cleave human plasminogen to generate angiostatin-like fragments, 
suggesting that specific kallikreins may have antiangiogenic actions. An 
understanding of the physiology of human kallikreins is emerging with potential 
clinical applications.