Serrapeptase and nattokinase are entirely different enzymes. They both are proteolytic enzymes, which is to say they both break down proteins, particularly fibrin in the bloodstream. They both originally came from natural sources in China and Japan, serrapeptase from silkworms and nattokinase from the fermented soy food natto. But they don't do quite the same things in the human body.
Serratiopeptidase breaks down collagen and fibrin. It can help the body “clean out” broken tissue in bones, muscles, tendons, and ligaments. It is especially useful for breaking down fibrin. This helps the body dissolve blood clots. That can be very helpful in healing bruises. And it prevents the formation of scar tissue so that joints retain their range of motion as they heal.
Nattokinase is partially broken up in the process of digestion. The pure nattokinase that passes into the bloodstream also dissolves the protein fibrin. This prevents or corrects some of the problems that are caused by blood clotting, but nattokinase does not break down damaged collagen.
Nattokinase fragments, however, can do something that serratiopeptidase cannot. They act as an angiotensin converting enzyme (ACE) inhibitor. They prevent the hormone angiotensin from being converted into the hormone angiotensinogen, which raises blood pressure. Just how much nattokinase lowers your blood pressure depends on how thoroughly it is digested. If you are taking antacids for heartburn, for instance, your stomach will produce less acid and the nattokinase will be less thoroughly broken down into fragments. It will have more of a blood thinning effect than a blood pressure lowering effect.
Some supplements combine serratiopeptidase with nattokinase. There are some people for whom this is a good combination. If you exercise as part of your routine for controlling your blood pressure and you suffer an injury, it is going to be harder to keep your hypertension in check while you are recovering.
Serratiopeptidase supports recovery of muscles, bones, and connective tissues, while nattokinase might give just that tiny boost in blood pressure control that makes the difference that keeps you from having to take more medication or adding a new medication. Don't take nattokinase if you take aspirin for your heart or if you are on prescription anticoagulant (blood thinning) medication. There is very little risk of an adverse reaction, only one adverse reaction reported in the medical literature, but the combined effects are impossible to predict.
And if you don't have high blood pressure, serratiopeptidase and nattokinase are redundant. Because serratiopeptidase is a stronger and more versatile proteolytic enzyme, and it doesn't just dissolve fibrin, it is the enzyme you need.