From Morton book:
The seeds contain 1.2% of the bitter, cyanogenic glycoside, lucumin; 0.0037% pouterin; 6.6% of a fixed oil; 0.19% saponin; 2.4% dextrose and 3.75% ash. The leaves possess an alkaloid, also resin, resinic acid, and a bitter substance.
Seeds also pretty hard, so could kill your blender before it kills you!
Sorry, that was wrong quote. It was for starapple instead of sugar apple. The sugar apple seeds are a lot more toxic than starapple seeds. You have to be VERY careful when juicing them that you don't accidentally leave one seed in the pulp and blend it. Here is the correct quote from Morton:
"The seeds are acrid and poisonous. Bark, leaves and seeds contain the alkaloid, anonaine. Six other aporphine alkaloids have been isolated from the leaves and stems: corydine, roemerine, norcorydine, norisocarydine, isocorydine and glaucine. Aporphine, norlaureline and dienone may be present also. Powdered seeds, also pounded dried fruits serve as fish poison and insecticides in India. A paste of the seed powder has been applied to the head to kill lice but must be kept away from the eyes as it is highly irritant and can cause blindness. If applied to the uterus, it induces abortion. Heat-extracted oil from the seeds has been employed against agricultural pests. Studies have shown the ether extract of the seeds to have no residual toxicity after 2 days. High concentrations are potent for 2 days and weaken steadily, all activity being lost after 8 days. In Mexico, the leaves are rubbed on floors and put in hen's nests to repel lice."