Enzymes can do some things that would be helpful in a fuel additive. Some can catalyze the breakdown of oxygen free radicals, which would inhibit fuel oxidation. Other proteins can bind metals that would otherwise accelerate oxidation. Lipoproteins could potentially act as water emulsifying agents, provided that could add enough to a mixture of gasoline and water.
But the problem is that the surface chemistry of enzymes requires an aqueous environment. They require water, not a hydrophobic solvent such as gasoline. Most proteins are not soluble in gasoline or even ethanol for that matter. Since undiluted Startron contains less that 0.5% active ingredients dissolved in a petroleum distillate carrier, you have to wonder how it could contain anything that would work when diluted into a tank of fuel.
Starton is manufactured by a company called Solpower Corporation using a technology developed by the Japanese Institute of Bio-Energy. If you do a search for that company, you will find some more information on the product than is provided by Starbrite. The enzymes are supposed to inhibit the agglomeration of asphaltenes, which are the oxidation products that form gum and varnish in stale fuel. They are also supposed to be able to dissolve small varnish particles if they are suspended in the fuel. Interestingly, they indicate that the product only stabilizes fuel for a maximum of 6 month in a vented tank. They also say that their product can be tested using the standard ASTM forced oxidation test, since enzymes are inactivated by high temperature. So you just have to take their word for it that the product will stabilize fuel.
There are proteins with enzyme activity that would be soluble in gasoline. Membrane proteins that contain attached lipid molecules would have such properties. But the enzyme activities would still require water for most of the reactions that they would catalyze. There may be exceptions such as enzymes secreted by oil degrading organisms. But that is a technology that is pretty far out there right now.
Scientists are necessarily septics. Unless empirical data are produced that can demonstrate the effectiveness of a product with seemingly "magical" properties, I won't be a believer. I can find no patents associated with this product, which increases my skepticism.