It is chemically the simplest vitamin. They are involved in hydrogen transfer. You should know the pathways or enzymes involved. Niacin can be synthesized from tryptophan. However, feeds low in niacin are usually also low in tryptophan.
Corn contains niacinogen which binds niacin tightly and makes it unavailable for absorption.
Deficiency in dog is called black tongue disease, in chicken is called spectacled eyes. Vitamin B6 is one of several vitamins still maintaining its original name. Synthesis of tryptophan from niacin requires B6. Vitamin B6 deficiency can precipitate niacin deficiency. Therefore, deficiency symptoms are similar for these two vitamins.