I take selenium, and I believe that by itself it is an antioxidant. I take it combined with a high dose of Vitamin E, as I have heard that it helps with Vitamin E absorption. There is a history of breast cancer in my family, and there are some studies that suggest that this combo may help. Also, if you have more fibrous (i.e., lumpy) breasts, it helps break the lumps down. My mother has been taking this combo too for the last 25 years, since she discovered a benign breast cyst at age 34. I am sure this is more than you ever wanted to know. But, this is my experience with Selenium. Since it is an antioxidant, I am sure it has value on its own.
I don't know about vitamin K.
Linda